How do you keep big printed panels from warping over months of outdoor parking?

It's both, honestly. Material matters, but the joining method is what keeps a body togehter long term. Glue alone between printed panels is not going to survive a summer on the road, especally ABS, which creeps in heat and has a big thermal expansion coefficient compared to steel or aluminum substructure. For exterior panels I'd look at ASA or a UV-stable PETG rather than ABS. ASA handles sunlight far better and warps less when printing large parts too. But the real answer is that printed plastic should never be the structural layer. Treat your printed panels as a mold or a core. Lay fiberglass or carbon cloth with epoxy resin on the inside of each panel, and ideally a skim coat of resin or filler on the outside too. Once the panels are skinned with composite, they stop behaving like plastic prints and start behaving like actual bodywork. Epoxy also bridges small seam gaps so your joints get much stronger. For the joints themselves, yes, mechanical fastening plus adhesive. Rivets or small bolts every few inches along the seam, with seam sealer or a flexible polyurethane adhesive like the stuff used for auto body panels between the pieces. The fasteners carry the load if the adhesive ever lets go, and the adhesive keeps water and vibration from working the fasteners loose. Dozens of panels glued only will open up at the corners first, always the corners.

On inspection, I went through a similar process in a state that requires a rebuilt or specially constructed vehicle inspection. The inspector cared about lights, brakes, steering, horn, mirrors, tires, seatbelts, and whether the thing looked professionally built and safe. Nobody pulled out a material spec sheet for my bodywork. What they did do was poke at things and look for sharp edges, loose panels, and anything that seemed like it would fall off at speed. If your panels are composite-skinned and fastened down like real bodywork, that impression goes a long way. Call the inspection station ahead of time and ask what they check, some states publish a checklist online and some will just tell you over the phone.

One practical tip: print test panels, join them, and leave the assembly on your dashboard in July for a month before committing to the whole body. Cheapest way to find out if your process survives real heat.

One thing I'd add on the print side: big flat panels are gonna curl no matter what filament you pick if you print them flat. I've had way better luck splitting a panel into strips and printing them on edge, then stacking them like brickwork with the seams offset. Warping gets way less angry that way, and the layers aren't all facing the sun the same direction either. Also don't skip paint. Even UV-stable filament chalks up after a couple seasons outside, a decent primer and topcoat buys you years.

I skinned a set of utility trailer fenders in PETG a few years back and the thing that saved them was slotted mounting holes. The panels grow and shrink at a diffrent rate than the steel frame over a hot day, and if every bolt hole is a snug fit the panel fights that movement until somethig cracks. Cut the holes oval, snug the bolts just enough to stop rattling, and let the adhesive do the sealing. My first attempt with tight round holes lasted exactly one summer before the corner split, the second set with slots is still on the trailer.

On the seam movement thing, butyl tape saved my rear hatch panels. I did rivets over butyl ribbon instead of relying on the adhesive to fill the gap, and two summers in the panels still sit tight with no cracking around the holes. It stays a little flexible forever so the panels can breathe without the seam opening up. Cheap roll from any auto parts store, stuck it on like double sided tape before I set the rivets.