# Repair and insurance glossary

> Terms that appear on repair estimates and insurance paperwork, defined plainly.

**Source:** https://ocrv.life/resources/glossary
**Business:** OCRV Center, 23281 La Palma Ave, Yorba Linda, CA 92887
**Phone:** (949) 799-3387
**Serving:** San Juan Capistrano, Orange County, California, roughly 26 miles from the facility

**Delamination.** Separation of the bonded layers inside a laminated sidewall or roof. The outer skin loses its grip on the foam core and the interior panel, so the wall flexes, ripples or bubbles under hand pressure. Water intrusion and adhesive fatigue cause most cases, and honest repair means peeling the assembly back to sound bonding rather than filling the surface.

**Laminated sidewall.** A wall built as a sandwich: exterior fibreglass or Filon, block foam with aluminium or wood framing, an interior substrate panel, then decorative wallboard. The layers are pressed together with adhesive under vacuum or roller pressure. The result is light, stiff and smooth, but it depends entirely on the bond line staying intact for its strength.

**Vacuum bonded lamination.** The factory process that presses a sidewall or roof sandwich together under atmospheric pressure inside a sealed bag or press table. Even clamping force across the whole panel produces a consistent bond line with no dry spots. Shop repairs reproduce it locally with vacuum bags or platen presses so the patched section matches original stiffness.

**Stick and tin.** Older trailer construction using a sawn wood stud frame with corrugated or smooth aluminium sheet screwed and stapled over it. There is no bonded laminate, so the skin carries almost no structural load. It dents easily but repairs cheaply, and a damaged section can often be unscrewed and replaced without disturbing the surrounding wall.

**Aluminium framed.** A wall or roof cage welded or riveted from extruded aluminium tube rather than lumber. It resists rot, weighs less than wood and holds dimension in heat, but it conducts cold through every member and telegraphs stud lines through thin skins. Damaged tubes are cut out and spliced with sleeved inserts sized to the original section.

**Fibreglass front cap.** The single moulded shell forming the nose of a motorhome or the front of a fifth wheel, usually gelcoated fibreglass laid up in a female mould. It carries the windshield opening, marker lights and often an overhead bunk. Caps are model specific, ordered from the coach builder, and frequently discontinued on units past ten years old.

**Unibody.** A construction method where the body shell itself carries structural loads instead of sitting on a separate ladder frame. Sprinter and Transit vans use it. Collision energy travels through designed crush paths, so repair requires sectioning at manufacturer specified locations and measuring the shell against factory dimensional data rather than eyeballing panel gaps.

**Dry rot.** Fungal decay in wood framing or subfloor that has been repeatedly wet and slow to dry. The wood darkens, crumbles under a probe and loses nearly all fastener holding power. It spreads along the grain well past the visible stain, so sound cut lines get established with a probe and moisture readings, not by appearance.

**Soft spot.** A section of floor, wall or roof that deflects underfoot or under hand pressure because the core beneath it has lost integrity. It is the field symptom that sends a technician looking for the leak path above it. By the time a soft spot is obvious, water has usually been present for at least one wet season.

**Subfloor.** The structural deck between the chassis and the finished flooring, typically laminated plywood or a foam cored panel bonded to steel or aluminium joists. It carries every appliance, cabinet and occupant load. Replacing it means pulling furniture, walls and sometimes slide rooms, which is why subfloor work is priced by section rather than by square foot.

**Moisture meter.** A pin or pinless instrument that reads moisture content in wood and composite panels. Pinless heads scan through finished surfaces without damage, which makes them the first tool used on a suspected leak. Readings get logged as a map across the wall or ceiling so the wet zone can be bounded before anything is opened.

**Luggage door.** An exterior compartment door on the lower body, usually a radiused aluminium or fibreglass panel on a piano hinge or a pair of gas struts. Coach builders use painted skins that carry the body graphics, so a replacement door almost always needs refinishing and decal work rather than a straight bolt-on swap.

**Basement.** The pass-through storage volume beneath the living floor of a diesel pusher or larger gas coach, framed between the chassis rails and often insulated and heated. It houses the wet bay, inverter, batteries and sometimes the generator, so basement collision damage is rarely cosmetic only and gets inspected for wiring and plumbing contact.

**Gelcoat.** A pigmented polyester or vinyl ester resin sprayed into the mould first, so it becomes the outermost surface of a fibreglass part. It is thick, colour saturated and repairable by sanding and buffing, but it chalks under ultraviolet exposure. Faded gelcoat can often be cut and polished back to gloss if enough film remains.

**Azdel.** A composite substrate panel of polypropylene and fibreglass used in place of lauan behind exterior skins. It does not absorb water, does not rot and weighs less than plywood, which is why builders advertise it. It is not structural on its own, so a wall using it still relies on the bond line and the framing.

**Filon.** A brand of fibreglass reinforced plastic sheet supplied in rolls and bonded to sidewalls as the exterior skin. It is thinner and flatter than a moulded cap and comes in gelcoated or paintable finishes. Because it arrives flat, it shows every imperfection in the substrate behind it, so prep quality decides how the repair reads.

**Lauan.** A thin tropical hardwood plywood, typically an eighth of an inch, used for decades as the interior substrate in laminated walls and ceilings. It is cheap, light and takes adhesive well, but it delaminates and rots quickly once wet. Finding failed lauan behind an intact skin usually widens a repair area well beyond the original estimate.

**Substrate.** Whatever layer sits directly beneath the surface being repaired or refinished. In body work it is the panel taking filler and primer. In lamination it is the backing sheet bonded to the skin. Identifying the substrate correctly decides adhesive choice, primer choice and whether a coating will actually stay stuck for years.

**Lap sealant.** A thick, non-sagging sealant applied over exposed edges and fastener heads at roof penetrations. It is meant to be tooled into a bead that stays where it is put on vertical and sloped surfaces. It is not self levelling, and using it on a flat horizontal seam leaves ridges that trap water instead of shedding it.

**Self-levelling sealant.** A flowable sealant that spreads under its own weight to form a smooth, feathered edge on horizontal roof seams. It bonds to membrane and metal flanges without tooling. Product chemistry must match the membrane, since a sealant formulated for EPDM can attack a TPO surface and lift the very seam it was applied to protect.

**EPDM.** Ethylene propylene diene monomer, a synthetic rubber roof membrane bonded over a roof deck. It is flexible, forgiving of movement and easy to patch, and it chalks white as it ages, which is normal rather than a failure. It is thinner than most owners expect and punctures under a dropped tool or a low branch.

**TPO.** Thermoplastic polyolefin membrane, the common replacement for EPDM on newer coaches. It holds a brighter white, resists chalking and can be heat welded at seams rather than relying only on adhesive. It is stiffer than rubber, so it telegraphs deck irregularities and demands a flatter roof surface underneath before it is bonded down.

**Fibreglass roof.** A one piece moulded or laid-up fibreglass roof panel used on higher end coaches and many fifth wheels. It resists hail and branch damage far better than membrane and can be buffed and gelcoat repaired. The tradeoff is weight, cost, and repairs requiring resin work and refinishing rather than a patch and a bead of sealant.

**Roof membrane.** The waterproof sheet covering a roof deck, bonded down and terminated under trim rails at the edges. It does the actual sealing; the deck below only provides shape and strength. Membrane rarely fails in open field. Sealant at vents, skylights and the front and rear terminations fails first, and the membrane gets blamed for it.

**Butyl tape.** A soft, permanently tacky sealing tape laid between a component flange and the exterior wall before the screws go in. Windows, vents, marker lights and trim rails all bed on it. It never cures hard, so it keeps sealing as panels move, but it squeezes out and thins across years of heat cycling.

**Wiper seal.** A flexible blade seal mounted around a slide room opening that sweeps debris and water off the slide face as the room travels. It is the first line of defence against grit reaching the mechanism. Worn wipers curl away from the wall and stop contacting the room, which shows as a dirt streak down the sidewall.

**Bulb seal.** A hollow compressible seal that squashes flat when a slide room is fully retracted or extended, closing the gap between room flange and sidewall. Because it works by compression it depends on correct room alignment. A room sitting a quarter inch low leaks past a perfectly sound bulb seal every time it rains hard.

**D seal.** A seal extruded in a D-shaped cross section with a flat adhesive backing and a rounded compression face. It is used on entry doors, compartment doors and slide perimeters where a bulb profile would sit too tall. Adhesive backed D seals fail at the adhesive long before the rubber wears out, especially on sun-facing walls.

**Blend panel.** An adjacent panel that receives a fading application of new colour so the eye never meets a hard line between old and new paint. Blending is not optional on metallics, where flake orientation shifts with every gun and every batch. The blend panel gets cleared edge to edge even though only part of it carries base.

**Spectrophotometer.** A handheld instrument that reads a painted surface at several angles and returns a numeric colour value. Matched against a paint manufacturer database it produces a mixing formula for the vehicle as it exists today, faded and all, instead of the code it left the factory with. It is the starting point, not the final answer.

**Sprayout card.** A test panel sprayed with the mixed formula, cleared and dried under the same conditions as the job, then held against the vehicle in daylight. It is the only honest verification that a formula matches. Adjusting a mix from a sprayout costs minutes; discovering the mismatch after a cap is painted costs a full redo.

**Tri-coat.** A three stage finish built from a ground coat, a translucent mid coat and clear. Pearl whites and candy reds use it. Colour depth depends on how many mid coat passes are applied, so matching means counting passes on a sprayout rather than trusting the formula. Tri-coats always consume more paint hours than a solid or metallic.

**Downdraft booth.** A sealed spray enclosure that pushes filtered air from ceiling plenums down past the vehicle and out through floor grates. The vertical airflow carries overspray away from the wet surface instead of across it. Downdraft is why a booth-sprayed panel has fewer inclusions than one shot in an open bay, and it controls cure temperature.

**Primer surfacer.** A high build primer sprayed over repaired areas to fill sanding scratches and minor low spots, then blocked flat before colour. It is the layer deciding whether a repair reads straight in reflected light. Skipping block sanding on a long RV panel produces waves that only appear once clear goes on and the surface turns glossy.

**Sealer.** A thin, non-sanding coat sprayed over primer and old finish to give base coat a uniform colour and porosity to lay onto. Tinted sealers reduce the number of base coats needed for coverage on strong colours. Sealer also isolates dissimilar underlying coatings so solvent from the topcoat cannot lift what is beneath it.

**Clear coat.** The unpigmented topcoat providing gloss, depth and ultraviolet protection over base colour. Nearly all polishable film thickness lives here. Clear failure looks like peeling or milky patches and cannot be buffed out, because once ultraviolet has broken down the resin the only correction is sanding back and refinishing the whole panel.

**Orange peel.** A dimpled surface texture in cured paint caused by atomised droplets not flowing together before the film sets. Gun setup, reducer speed and booth temperature all drive it. A small amount matches most factory finishes. Heavy peel is sanded flat with fine abrasive and polished, which costs film thickness and therefore has a hard limit.

**Fisheye.** Small round craters that open in wet paint where surface contamination repels the coating. Silicone from tyre dressing, wax residue and compressor oil are the usual causes. A fisheye cannot be filled by spraying more material over it. The panel gets sanded back past the crater, cleaned properly and resprayed, so booth hygiene matters.

**Solvent pop.** Pinholes that appear when solvent trapped under a skinned-over film boils out during cure. Heavy coats, short flash time and premature baking cause it. The defect goes all the way through the coating, so repair means sanding to a sound layer and refinishing rather than polishing. It is a process failure, not a material defect.

**Flash time.** The dwell between coats that lets solvent evaporate before the next pass goes on. It varies with temperature, humidity, film build and reducer speed, and it is the step most commonly shortened when a shop is rushing. On a long RV panel the flash also lets a painter walk the length of the coach checking coverage.

**Wet sanding.** Sanding cured clear with fine abrasive and water or lubricant to flatten texture, remove dust nibs and level a repair into surrounding finish. Grits typically run from 1500 through 3000 before compounding. It consumes clear coat film, so it is measured against a gauge rather than done by feel on a panel expected to last.

**Mil thickness.** Coating depth measured in thousandths of an inch with a magnetic or eddy current gauge. It tells a technician how much clear is available before cutting through, and it reveals prior repairs on a used coach as thickness spikes on individual panels. Readings are taken before correction work so polishing limits are known up front.

**Oxidation.** Surface degradation of gelcoat or clear from ultraviolet exposure, leaving a chalky, colour-dead layer that transfers to a rag. On gelcoat there is usually enough material to cut back to bright colour. On clear coat there often is not, which is the fork in the road between a paint correction job and a refinish job.

**Schwintek.** An in-wall slide mechanism using vertical toothed rails driven by two synchronised motors mounted in the side jambs. It saves interior floor space, which is why it appears on bedroom and wardrobe slides. Synchronisation is electronic, so a room walking out of square is usually a controller or motor fault rather than a bent rail.

**Slide topper.** A spring loaded fabric awning mounted above a slide room that unrolls as the room extends and keeps debris and standing water off the slide roof. Toppers stop water reaching the seal, but a sagging or ponding topper does the opposite. Fabric gets replaced far more often than the roller assembly it winds onto.

**House battery.** The deep cycle bank powering twelve volt lights, pumps, fans and control boards when a coach is off shore power. It is separate from the chassis starting battery and is built for sustained discharge rather than high cranking current. Bank capacity, not inverter size, decides how long an appliance can actually run overnight.

**AGM.** Absorbed glass mat lead acid batteries, where electrolyte is held in fibreglass separators instead of sloshing freely. They are sealed, tolerate vibration and mounting angle, and need no watering. Usable capacity is roughly half of rated amp hours before cycle life suffers, and that is the number that matters when sizing a bank.

**LiFePO4.** Lithium iron phosphate chemistry, now the default upgrade for house banks. It delivers most of its rated capacity, holds voltage flat under load and cycles thousands of times. Converting a coach is not a drop-in swap: the converter, the alternator charging path and any solar controller all need charge profiles the chemistry accepts.

**Converter.** The unit taking 120 volt shore or generator power and producing regulated twelve volt output to run house loads and charge the battery bank. Older single stage converters hold a fixed voltage and slowly cook lead acid batteries. Multi stage units step through bulk, absorption and float, which is what most battery upgrades actually require.

**Inverter.** A device converting stored twelve or twenty four volt battery power into 120 volt alternating current for outlets and appliances. Pure sine output matters for anything with a motor or a switching supply. Inverter chargers combine the function with a converter in one chassis and add a transfer relay, which simplifies the wiring considerably.

**Transfer switch.** An automatic relay assembly deciding whether shore power or generator output feeds the coach panel, with a deliberate delay so the two sources are never tied together. Burned transfer switch contacts are a common cause of half a coach losing power, and the symptom often gets mistaken for a failing generator or a bad pedestal.

**Shore power.** Utility electricity supplied to a parked coach through a cord, as opposed to power made on board by a generator or inverter. Everything downstream assumes a clean, correctly wired source. Reverse polarity, an open ground or low voltage will damage air conditioners and electronics well before anything inside the coach trips a breaker.

**Shore power pedestal.** The campground post carrying the breakers and receptacles a coach plugs into. Pedestal condition varies enormously between parks and is outside anyone's control. Worn receptacle contacts create heat and voltage drop under load, which is why a surge protector reading voltage and wiring faults earns its cost on the first bad post.

**30 amp service.** A single 120 volt leg rated at thirty amps through a three prong TT-30 plug, giving roughly 3,600 watts total. It runs one air conditioner comfortably and very little else at the same time. Load management on a thirty amp coach is a scheduling exercise: microwave and air conditioner take turns rather than sharing.

**50 amp service.** Two 120 volt legs at fifty amps each through a four prong plug, delivering around 12,000 watts. It supports multiple air conditioners, a residential refrigerator and cooking loads at once. Legs are balanced across the panel at the factory, so adding a large load without checking which leg it lands on trips one side repeatedly.

**MPPT.** Maximum power point tracking, a solar charge controller design converting excess panel voltage into additional charging current. It typically harvests twenty to thirty percent more energy than a simple controller, and it allows panels to be wired in series with thinner cable runs. It costs more and suits almost any full roof array.

**PWM.** Pulse width modulation, the simpler solar controller type that switches the panel connection on and off rapidly to hold battery voltage. It effectively drags the panel down to battery voltage and discards the difference. On a small trickle array it is perfectly adequate. On a large roof array it wastes a meaningful share of available energy.

**Hard start capacitor.** An added capacitor boosting starting torque on an air conditioner compressor so it comes up to speed faster. It shortens the inrush spike but does not lower its peak by much. It is a cheap fix for a compressor struggling on a weak supply, not a way to run two rooftop units on thirty amp service.

**Soft start.** An electronic module ramping an air conditioner compressor up over a short window instead of throwing it straight across the line. It cuts starting current substantially, which is what makes running a rooftop unit from an inverter or a small generator realistic. It does nothing for running amps once the compressor is turning.

**Absorption refrigerator.** A refrigerator with no compressor that moves heat using an ammonia solution driven by an LP flame or an electric heating element. It runs silently and works off grid on gas alone, but it cools slowly, dislikes being out of level, and eventually fails when the cooling unit leaks, which is a replacement rather than a repair.

**Residential refrigerator.** A conventional household compressor refrigerator fitted into a coach, now common on larger units. It cools faster, holds temperature better and does not care about being level. It also runs only on 120 volt power, so it requires an inverter, enough battery capacity to carry it overnight, and secure mounting against road vibration.

**Hydronic heat.** A central heating system warming a glycol loop with a diesel or LP burner and circulating it to fan coil units through the coach, often supplying domestic hot water from the same boiler. It heats evenly and quietly. Diagnosis differs from forced air work because faults hide in circulation pumps, zone valves and trapped air.

**Sail switch.** A small airflow paddle inside a forced air furnace that must be pushed open by the blower before the control board allows ignition. It is a safety interlock preventing a burn without airflow. Dust, a weak blower motor or a restricted duct all keep it from closing, producing a furnace that runs the fan and never lights.

**Wet bay.** The exterior service compartment holding the city water inlet, fresh fill, tank valves, low point drains and often the water pump and filter. Everything plumbing related is manifolded here for access. Because it sits low and outboard it is exposed in curb strikes and low speed collisions, and freeze damage concentrates here on stored coaches.

**Black tank.** The holding tank receiving toilet waste, fitted with a three inch gate valve and a level sensor set. Sensors misread constantly because solids cling to the probes. Tank failures show up as cracks at the mounting straps or at the flange where the toilet ell meets the tank, and both require dropping the tank to repair properly.

**Gray tank.** The tank collecting sink and shower drainage. It fills faster than most owners expect because shower use dominates it. Grease and hair build a film that fouls level sensors and slows the drain. On coaches with two gray tanks the galley tank and the bath tank often share one dump outlet through a Y fitting.

**Macerator.** An electric pump with a cutting impeller that grinds waste and pushes it through a small diameter hose, allowing a dump uphill or over distance. It removes the need for a gravity drop but adds a motor that jams on anything not designed to break down. Rebuild kits exist, and the impeller is the usual failed part.

**ADAS.** Advanced driver assistance systems: forward collision warning, lane keeping, adaptive cruise, blind spot monitoring and automatic emergency braking. Sensors sit behind the windshield, in the grille and in mirrors and bumpers. Any body repair moving a sensor mount or replacing glass puts aiming out of specification even when nothing looks different.

**ADAS calibration target.** A printed board or reflector array positioned at manufacturer specified distances and heights in front of a vehicle so a camera or radar can relearn its aim. Static calibration demands a level floor, controlled lighting and precise measurement from the thrust line. It is a shop procedure, and the setup takes longer than the scan.

**Dynamic calibration.** A calibration completed by driving the vehicle at a specified speed on marked roads while the module learns from real lane lines and traffic. Some platforms need only this; many need a static setup first and a dynamic run afterwards. Weather, lane paint quality and traffic density decide whether the procedure completes on the first attempt.

**Pre-scan.** A full module health scan performed at intake, before any part comes off. It records every stored and active fault so damage-related codes are documented separately from pre-existing ones. Without a pre-scan there is no evidence a fault came from the loss, which is exactly the argument that stalls electronic repair lines on a claim.

**Post-scan.** The verification scan run after reassembly and calibration, proving no faults remain and every module reports ready. It closes the loop opened by the pre-scan and becomes part of the delivery file. On vehicles with driver assistance it separates a vehicle that looks repaired from one demonstrably back to specification.

**Betterment.** A deduction a carrier applies when a repair replaces a worn item with a new one, arguing the owner ends up better off than before the loss. Tyres, batteries and awning fabric attract it most often. It is negotiable, and the argument turns on documented remaining service life rather than on the age of the part alone.

**Diminished value.** The market value a repaired vehicle loses purely because it now carries an accident record, even when the repair is faultless. Recovery depends on state law and on whether the claim is first party or third party. Documentation of pre-loss condition and a credible appraisal move the number, not the size of the repair bill.

**LKQ.** Like kind and quality, the label for used or salvage parts written into an estimate as an alternative to new components. On discontinued RV caps and compartment doors it is sometimes the only path to a repair at all. Condition of the specific donor part matters far more than the abbreviation implies, so it gets inspected on arrival.

**Supplement.** An amendment to an approved estimate covering damage found after teardown began. On RVs supplements are the norm rather than the exception, because laminated construction hides water and impact damage behind an intact skin. A supplement submitted with photographs and measurements moves faster than one submitted as a bare line item request.

**Teardown.** The controlled disassembly of a damaged area to expose everything an initial visual estimate could not see. It converts guesses into measurements. On a laminated wall or a slide room the teardown routinely doubles documented damage, which is why carriers and shops both prefer authorising teardown before arguing about a repair total.

**Appraisal clause.** A provision in most policies allowing either side to demand a binding valuation when they disagree on the amount of loss but not on coverage. Each party appoints an appraiser and the two select an umpire. It resolves value disputes without litigation and is frequently overlooked by owners facing a low total loss offer.

**Total loss threshold.** The point at which a carrier declares repair uneconomical, usually expressed as repair cost plus salvage against actual cash value, with state rules setting a floor. RVs reach it slowly because values run high, but a water damaged coach can cross it on interior scope alone even with an undamaged exterior shell.

**Actual cash value.** Replacement cost less depreciation, which is what most policies pay on an older unit unless agreed value or replacement cost coverage was purchased. Comparable sales drive the figure, and RV comparables are thin, so the initial valuation report is worth reading line by line for the wrong floor plan or the wrong model year.

**Deductible.** The portion of a covered loss the owner pays before coverage responds, chosen at policy inception. Comprehensive and collision deductibles are often set at different amounts, which matters when a claim could be written either way. It is collected at delivery, and no shop may lawfully absorb it into an inflated repair figure.

**OEM parts.** Components supplied by or for the original manufacturer, carrying factory fit, factory finish and factory part numbers. On coach caps, slide mechanisms and appliance bezels the fit difference is real and measurable. Policies vary on whether they pay for them, and older RV components are often discontinued regardless of what a policy allows.

**Aftermarket parts.** Components made by a third party to replace an original. Quality spans a wide range, from excellent to unusable. On RVs many so-called aftermarket parts are the same supplier component the coach builder bought, since builders rarely manufacture their own vents, doors or mechanisms. Fitment gets verified before a panel goes into prep.

**Direct repair program.** An arrangement where a carrier refers claims to a participating shop in exchange for agreed pricing and process concessions. Owners are never obliged to use one. An independent facility bills the same carriers directly without the referral agreement, which keeps repair decisions with the shop and the owner rather than a program administrator.

**Loss of use.** Coverage for the cost of substitute transportation or lodging while a vehicle is out of service. RV policies often cap it at a daily figure and a total number of days that assume an automotive repair timeline. Cap and parts lead times on coaches routinely exceed that window, so the limit is worth checking at first notice.

**GVWR.** Gross vehicle weight rating, the maximum a single vehicle may weigh fully loaded, set by the manufacturer and printed on the federal placard. It is a design limit covering brakes, tyres, suspension and frame together. Exceeding it is a structural and legal problem, and it is the number every other weight figure gets measured against.

**GCWR.** Gross combination weight rating, the maximum allowable weight of tow vehicle and trailer together with everything in both. It is limited by drivetrain and cooling capacity of the tow vehicle, not by the hitch. A combination can sit under both individual ratings and still exceed the combination rating, which is the common towing mistake.

**GAWR.** Gross axle weight rating, the maximum load a single axle may carry, listed separately for front and rear. Axles get weighed individually on a scale because a coach can sit under its total rating while overloading one axle badly. Slide rooms, generators and water tanks concentrate weight and skew distribution more than cargo does.

**UVW.** Unloaded vehicle weight, what a unit weighed leaving the factory with standard equipment, full fuel and engine fluids but no water, propane, cargo or occupants. Dealer installed options are not in it. It is the baseline for calculating carrying capacity, and it is frequently optimistic relative to what a scale actually reports.

**CCC.** Cargo carrying capacity, the weight remaining for belongings, water, propane and passengers after unloaded vehicle weight is subtracted from gross vehicle weight rating. Full fresh, grey and black tanks consume a surprising share of it at roughly eight pounds per gallon, which is why capacity should be calculated wet rather than dry.

**Dry weight.** A marketing figure for the weight of a unit with no fluids, no propane, sometimes no batteries and no options. It is useful for comparing brochures and almost useless for planning a load. Real world weights commonly run well above it once dealer additions, batteries and a full water tank are accounted for honestly.

**Wet weight.** The weight of a unit with fresh water, propane and waste tanks filled to their normal travelling state. It is the honest starting point for capacity planning because water alone can add several hundred pounds. Weighing a loaded coach on a certified scale before a long trip converts every estimate into one verified number.

**Tow rating.** The maximum trailer weight a given tow vehicle configuration can pull, published per drivetrain, axle ratio and cab arrangement. Advertised maximums assume a stripped vehicle with a driver and nothing else. Passengers, a loaded bed and a heavy hitch all come off it, so the usable figure is always lower than the headline.

**Tire load rating.** The maximum weight one tyre may carry at a specified cold inflation pressure. Multiplying by the number of tyres on an axle gives the axle capacity that must exceed measured axle load. Under-inflation is the leading cause of trailer tyre failure, and it destroys casings long before tread depth becomes a factor.

**Pin weight.** The downward load a fifth wheel trailer places on the truck bed hitch, generally fifteen to twenty five percent of loaded trailer weight. It transfers directly onto the truck rear axle and counts against that axle rating and the truck payload figure. Toy haulers shift pin weight dramatically depending on whether the garage is loaded.

**Tongue weight.** The vertical load a conventional trailer coupler applies to the hitch ball, usually ten to fifteen percent of loaded trailer weight. Too little makes a trailer sway; too much unloads the tow vehicle front axle and ruins steering. Moving cargo a few feet fore or aft inside the trailer changes it more than owners expect.

**King pin.** The vertical steel pin projecting down from a fifth wheel trailer pin box that locks into the jaws of a truck bed hitch. It carries the entire pin weight and every acceleration and braking load. Wear, elongation or a bent pin box shows up as a coupling that will not release cleanly, and it gets measured rather than eyeballed.

**Fifth wheel hitch.** The plate and jaw assembly bolted to rails in a truck bed that accepts a trailer king pin. It pivots on trunnions to allow pitch and roll. Mounting rails must land on the truck frame, and a rail installation that misses the frame or uses the wrong hardware is a structural defect no amount of hitch quality corrects.

**Weight distribution hitch.** A hitch using spring bars under tension to transfer part of the tongue load forward onto the tow vehicle front axle and back onto the trailer axles. Set up correctly it restores front axle weight and steering feel. Set up by guesswork it either does nothing or over-levers the coupler, and both conditions show on a scale.

**Sway control.** Friction bars, cam arrangements or integrated designs resisting yaw between tow vehicle and trailer. They damp oscillation once it starts but do not fix the cause, which is usually inadequate tongue weight or a badly distributed load. Electronic trailer sway mitigation in the tow vehicle brakes individual wheels to achieve a similar effect.

**Chassis rail.** The main longitudinal steel members running the length of a truck, motorhome or trailer frame, carrying every load into the suspension. They are usually C-channel or boxed section. Heat, drilling and welding all change their properties, so repairs follow specified splice locations and reinforcement patterns rather than whatever seems convenient at the bench.

**Outrigger.** A short lateral member cantilevered off a chassis rail to support body structure wider than the frame. Motorhome sidewalls, service bodies and slide room floors all land on them. They bend first in a curb strike or a low speed side impact, and one bent outrigger throws off body alignment along an entire side.

**Crossmember.** A transverse member tying two chassis rails together, setting frame width and resisting the twisting the rails cannot resist alone. Removing one to route plumbing or fit a tank without replacing the lost stiffness is a common conversion error. The consequence shows as body cracking and door fit problems rather than an obvious frame fault.

**Subframe.** A secondary frame structure carrying a specific assembly, such as engine and front suspension on a van or the body mounting structure on a cab chassis conversion. It bolts or welds to the main structure and is designed to deform in a controlled way. Straightening one is measurement work on a bench, not a pulling exercise.