Yeah, you are not the first to buy a decent looking rig and discovered a lot of water damage. I got lucky. I bought something that showed previous damage so I knew what my course of action was going to be. I also had the intention of insulating the structure so it could endure a Midwest winter with more comfort.
One of the better aspects of gutting the whole thing and starting over is that you can decide what fits your use and can design/build accordingly. I can't really convey what kind of improvement in comfort, feel, and lack of noise a really well insulated trailer is like. I live in MN so are summers aren't brutal but +90f and the sun shining is not atypical. At night, the temps go down. If I leave a vent cracked and a window open, at noon, the temperature inside the trailer is -10 to -15f from ambient. Last winter, I heated the TT with a small electric heater at it's lowest setting, 550w or so.
Yes, an interior wall partition or a full shower stall may be holding up the roof (especially true if the ends of the ceiling joists are rotten). What I did was build a jacking post out of 2x4 and raise said post with a small hydraulic jack. The link in my signature has pictures of how I repaired the ceiling joists and wall framing. Having an aluminum frame is a better deal, all of that should be in good shape.
If you can, please, post some pictures up of your progress. Having electrical skills is a definite +++++, you can do some upgrading of both the 120v and 12v systems.
I'm pullin' for 'ya!