Hopefully your floor is in reusable condition, or this becomes hopeless.
If I learned anything in life with home projects......don't waste any time trying to salvage the cheap stuff, that could be quickly removed and replaced with new.
My thoughts on this project is to rebuild (maybe replace) one or two adjacent walls at a time, and do the roof last. I wouldn't want to tear it down to the point that there is nothing to refer to anymore.
If this were my project, I would change the following for sure
- reshape the cab-over bed for improved aero-dynamics
- have only one small screened window on each side of the cab-over bed. Delete the front window for sure for water infiltration reasons
- replace every window in the entire RV with new ones. You can pick different sizes too.
- finish the RV in sheet fiberglass including the roof. forget the siding and rubber or aluminum roof
- If the roof needs replacing, crown it to eliminate water puddling later which always happens around the heavy a/c unit. When the caulk cracks around the a/c and other roof openings, in comes the water
- use 0.6 CCA treated lumber (or a safer equivalent) for all framing alone, not the more common 0.4 CCA. CCA Pressure treated lumber is a great product but has health concerns. Limiting it's use to framing alone minimizes any concerns.
- use moisture-proof exterior wall board sheathing. What kind needs further research.
- use hurricane ties or other steel bracketry for all frame corner joints. These are commonly found at Home Depot for house and deck construction.
- screw all wood together with high strength coated wood screws. Stainless screws don't rust, but they are a very soft metal and will break easily from the stresses that occur during RV motion. Pre-drill the screw holes near the ends of any framing to prevent splitting
- rewire everything with 12 gauge wire for 110v, etc. All wiring in there is likely too old with brittle insulation for continued long term use.
Regardless of condition, be sure to replace or spray bleach on all good wood that had mold grown on it. You wouldn't want to have health problems from this.