I got a pop up jayco 12' as pay for a mechanic gig last year, after a overall check, it's got a lot of water damage/rot in the base walls, it's going to need rebuilt.
Good thing, the fridge, propane stove, heater & rooftop a/c work fine.
Going to have to tear down to frame & rebuild with 5'6" walls, so it won't be a pop up camper anymore.
Hadn't yet decided if want use 1" square tube stock & weld up walls & roof frame or rip 2x's into 1 5/8" x 1 5/8" "studs".
I do already have plenty of painted .030 4'x8' sheetsteel for outter skin & will rip strips for seams and bend 90s for corner beads.
whether I use square stock or lumber, I'll get whatever thickness styrofoam sheets and insides will use 1/8" 4'x8' wood paneling or tempered hardboard, might even use the 1/16" 4x8 sheets of bath/shower laminate if find cheap enough.
The stove, heater, fridge & a/c will go back in.
it's a project.