....in the factory visits I've done watching the truck camper construction process, all of them build from the tub up (the tub is built, then the under-wings; all cabinets are added just after walls are connected)...hmmm?
...in my mind, a full foot wall framing (or 2x6 x) would probably need to be built just under the wings and as far rear as possible on both sides, and stabilized against collapse; then the camper lowered onto this foot wall.
You need to work up a real 3D CAD model, with the structure, and do fea runs. The problem is: you don't know where the weak parts of the structure are "hidden", let alone what the structure actually is under "the skin" ! Even worse, the camper floor has to be removed.....by someone working under the camper (extremely dangerous). And, the entire guts of the camper (all furniture, fixtures) have to be removed from the inside 1st (they are all attached to the floor on the inside).
Why even pull the floor:
You really need a structural engineer to have a look, and possibly suggest "sistering" some kind of full camper-length beams UNDER the existing floor, then lowering camper onto the beams (the camper weight straightening out the floor), then with some manner of understructure, connecting the new sister beams to the camper with large lag bolts.
Just build another shell:
What about building a new camper from scratch (and gut all the current appliances and jacks and electrical, and install in your newly-built unit?)...the appliances, electricals, cabinetry, shower parts, toilet, kitchen accouterments and windows and door and roof-top vents are the most expensive components. The shell can be built relatively inexpensively (you could probably frame up the new camper in 2 weeks; then rent a sprayer, and coat the entire plywood structure in water-based quick dry spar varnish several times in 1 day, then attach aluminum cladding from your old camper, and buy what you need to finish the skinning)....?
....just thinking allowed...
I remember reading about a Frank Lloyd Wright house (Falling Water) balcony system sagging down into the river below; the fix was to introduce tensioner cables, then tension them to lift the sagged balcony...
