Wall studs are generally kiln dryed 2X2 lumber. Roof joists, 2X4s cut with a taper for water run off. Floor joists, again I would go with 2X4s for strength. Since your stripping it and plan on a total rebuild, make a plan, identify what you might like to have where and run new wiring placing stuff where you want it. Easy now, not so easy once the wall paneling is in. As for interior paneling, you can use luan plywood, or buy some prefinished paneling and glue it on. As for what to use? Pick what you think works best. Insulation? I have always thought spray foam would make a tighter RV. Falling apart? Contact some spray foam contractors and ask them. Their the experts and should have the answers.
