What we take in, from a teardrop to a diesel pusher

Two vehicles that look similar from the kerb can be built nothing alike underneath. A motorhome sidewall is usually a sandwich: an outer skin bonded to rigid foam over a thin inner substrate, carrying load across its whole face rather than through a frame behind it. A truck cab is stamped steel over a ladder frame, designed to fold in a defined order and to be cut and replaced at defined points. The first is repaired by restoring the bond and the panel as one structure. The second is repaired by measuring the frame, pulling it back to published tolerance, and replacing what the builder intended to be replaced.

That difference is why the number on a coach moves so much after teardown. On a bonded panel, damage travels. A hit at the rear corner separates the bond several feet away from the impact, and the separation stays invisible until the skin comes off. On a steel cab the damage mostly stops where the energy stopped. An owner of a bonded vehicle should treat the first estimate as provisional and plan for a supplement. An owner of a framed metal vehicle usually sees a first estimate that holds.

• Bonded sandwich construction. Coach and trailer sidewalls, slide faces, most fiberglass caps, roof laminates.
• Framed metal construction. Truck cabs, box and reefer bodies, bus skirt panels, van shells.
• Both at once. A conversion is a metal shell with a bonded structure built inside it, and the two repair standards meet somewhere behind the wall panel.

Van and truck chassis builders publish body repair manuals: section sizes, weld types, permitted sectioning points, what may be pulled and what has to be cut out. Work on a Sprinter shell or a current production electric van is measured against a document somebody wrote on purpose. Most recreational vehicle manufacturers published nothing of the kind. There is no sectioning diagram for a fifteen year old fifth wheel sidewall, no torque table for a cap, and frequently no drawing at all of what is inside the wall the technician is about to open.

Where a procedure exists we follow it and the file records that we did. Where none exists the standard falls back to the chassis builder's structural tolerances, measurement against those tolerances, and trade practice for composite repair, which is why structural work here is measured and photographed rather than judged by eye. It also explains a cost difference owners find backwards. A documented body is often cheaper to repair correctly than an undocumented one, because nobody is spending bench hours discovering how it was assembled in the first place.

Most motorhomes, shuttle buses, box trucks and vehicle based business builds are two vehicles wearing one paint scheme. A chassis builder made the rails, the cab and the running gear. A separate coach or body builder mounted everything above it, often months later and often to a specification that changed partway through a production run. That split runs straight through the parts channel. Chassis parts come through a truck dealer network with a part number and a delivery date attached. House side parts come through the body builder, through the original component supplier, or through nobody at all.

The practical test for an owner is short. Look at what is damaged and decide which half of the vehicle it belongs to. A bumper, a headlight, a mirror, a step, a fender: chassis side, and usually orderable. A cap, a sidewall section, a compartment door, a slide face, a window frame: house side, and the honest answer may be fabrication rather than a purchase order. Fabrication is not a downgrade. On a discontinued cap it is regularly faster and better fitting than a used part somebody located three states away and shipped on a pallet.

Two identical dents on two different vehicles can be a two week job and a ten week job. What separates them is almost never labour hours. It is whether the damaged part is a current production item, a superseded item, or a shape that has to be built from scratch on a bench. The longest waits in this shop are not attached to the largest repairs, and telling an owner otherwise at intake would be a comfortable lie that falls apart in week three when the coach is still sitting there.

• Fastest. Current production vans, trucks and chassis components, where a dealer network already stocks the part.
• Middle. Current model coaches and trailers, where the body builder still supplies the panel but ships on a schedule of its own.
• Slowest. Obsolete caps, discontinued sidewall profiles, aluminium skin sections, and anything from a builder no longer trading.
• Least predictable. Agency specified equipment on a public fleet vehicle, where the specification has to be matched exactly and the supplier may be the only one.

Three questions get an owner most of the way to an accurate expectation. What is the body made of at the point of damage, metal or bonded composite. Who built the damaged part, the chassis maker or the coach builder. Is the vehicle still in production. Answer those three and you can predict the shape of your own repair before anyone writes a line, including whether the number is likely to move once panels come off. Arrive with the answers, or with photographs from each corner and a close view of the damage, and the first conversation starts several steps further along than it otherwise would.