Welcome to the forums! Lots of friendly and knowledgeable folks here! Someone better able to address your specific question(s) will be along shortly.
I’ll get the ball rolling with some general thoughts. It will simplify things for you if you put the year, manufacturer, and model in your signature. Also, one topic per post is best. Often one topic turns into several related topics. That becomes very confused if your original post asks both a plumbing and electrical question. Pictures almost always help us understand whatever you’re asking about.
You’ve certainly got yourself quite the project. If you don’t have someplace indoors to park it while you work on restoring it, you should probably get some tarps on the roof. Also, if the HVAC is operable, getting it plugged in will make it much more comfortable to work on. It will also provide power for plugging in tools.
You’ll need to determine damage and structural soundness from the inside. And that needs to be determined before getting on the roof. I’m imagining damage to walls with black mold. Depending on year and make, your structural supports may be wood or may be aluminum. I would think exposing all obviously damaged areas (removing inside walls) would be your starting point. A complete assessment of what you’re up against is needed to determine if it’s worthwhile to proceed. (Even if you’re doing all the work, it may be too far gone to make it financially reasonable.) I’ve never done anything like this; somebody more experienced will be along shortly.
From your initial description, you’re probably going to need a dumpster… there’s going to be a lot to get rid of.
Best of luck!
edit: I see Rene has beat me to te first reply. Apologizes if I’m parroting some of his thoughts.