I gutted/restored an old Starcraft. The build thread is in my signature line,
The Cowboy/Hilton. Plenty of discussion and pictures in that thread.
To answer some of your questions:
1) Use the same size wood that is found in the rest of the frame. There is no standardization.
2)Azdel is a plastic composite and you'll pay a lot for it, if you can even buy it. I used conventional SPF lumber for most of the frame. I think I had a couple of treated boards that found their way into top plates. It is not advised to use it in interiors but I sealed everything up real tight. For interior paneling I used Luan board, sealed on both sides and all edges.
3)Your siding appears to be aluminum but may be fiberglass. I used about 40 tubes of construction adhesive and various sealants. Most framing members were glued and screwed together. Construction adhesive will bond most anything. I have aluminum siding so chose to use a 1" fiberglass batt against the siding. That allowed for in-wall wires and a place for moisture to egress. I followed that up with a 1" extruded foam board in the wall cavities.
4)#2 construction grade spray foam is the best insulation you can buy. It will not "crumble". It is also about the most expensive. I chose to cut and friction fit foam board. I also used a 1/4" foam board across the studs and a poly vapor barrier. That two weeks of work was totally worth it. I enjoy my comfortable trailer every day.
5)Yes, I would plan to replace the wiring. You will be able to install a better quality of work and a better layout than what a mfg does.
6)No particle board, please. That would include Masonite or other paneling made from wood particles. The inside of a trailer can get very humid and condensation could destroy particle board. As mentioned, I used Luan board, mostly for it's finish aspects under a clear sealant. A good grade of plywood would work as well and if you plan to paint the interior, plywood would be a good choice.
If I had a couple of tips to give, they would be: Draw out plans for wiring, plumbing, and fixture locations. They don't have to be architectural grade but a drawn plan that lists materials will mean that you don't have to rework an area because of poor schedule or forgetfulness.
Second, allow enough continuous time to complete the RV restoration. There are a lot of half-completed RV projects out there. I did take breaks in the schedule to accomplish work but, for the most part, stayed at it until done (if there ever is a "done", lol). It ate up a good chunk of 6 months.
These projects are always worthwhile because a guy can end up with a result that is customized to his use. If you do the insulation as good as possible and are able to replace most windows with Low-E glass, you will be astounded on the difference in comfort and sound. I can barely hear traffic outside my trailer and keeping cool or warm is an easy task.
Good luck and let's see that pile of debris!
