I've been there but had an old Starcraft to start with instead of a FEMA. Yours can't be any worse than mine. I could put my arm through certain parts of the siding and see the road through the wheel wells.
All of the renovation is in this thread:
The Cowboy/Hilton.
When planning any work like roof repair or wall repair, I'd suggest to tear it apart slowly to see what you have. My roof (aluminum) leaked in many locations and some roof trusses were rotted at the ends, not supporting anything. I also had a lot of framing to repair. I didn't need to replace the whole roof because it only leaked at the edge joint to the walls (typical). The roof was resealed by using Eternabond tapes over all the seams. If you find it's necessary to remove the whole roof membrane and replace a lot of the roof decking, I'd suggest to replace the whole membrane with a new piece of EPDM (rubber). That will be cheaper and more durable than the liquid stuff.
If I had one piece of advice to give you it would be to draw out plans for framing, plumbing, electrical, and how you want to locate everything else. The plans don't have to be anything special but it will help a lot when things start going back together. I only missed an outlet for the fridge and covered a wall switch location but it's no fun tearing out your new work when you find something's missing or a new cabinet doesn't fit. The drawings will also alert you to the schedule of what needs to go back in next or before something else gets done.
Some things learned:The factory work is sloppy. I've built better doghouses than some of my trailer. When the mfg can cut a corner or use an inferior material, they will. Insulation and new triple pane windows pay for themselves in energy costs and add greatly to the comfort. Watch your weights, you may add some when renovating and you want it balanced at the end.
The really great thing about stripping and rebuilding is that you can design the interior just the way you want it. Since you will be adding holding tanks, you can locate plumbing and fixtures anywhere you wish. The same thought goes for cabinetry and beds, locate them where you want them. Add countertops, there is never enough.
If you have any questions or need some help, post it up here and add some pictures of what you're doing, please. If you have complex questions, send me a PM, I'll be happy to help.
Best of luck with your project.