I restored a 22' trailer. The pictured thread is in my signature line,
The Cowboy/Hilton. I did pretty much what you are planning. It was a very worthwhile project as I was able to incorporate some ideas that made the trailer as comfortable and efficient as I wanted.
The recovery work with the framing was at about the level of someone that can build a half-way decent doghouse. I have been a carpenter for many years so that part of the restoration was painless.
I'd suggest that you plan to remove the interior paneling to discover any water intrusion/frame repair. New Luan paneling is still rather inexpensive and will be a good surface for wall coverings or paint. I just applied a varnish finish on the paneling, it was part of my "bunkhouse design" criteria.
I also upgraded the 120V service to 50amp service. There was nothing very good about the original 120V distribution and 50 amp was no cost penalty until you buy the larger shore cord and power inlet.
I'd suggest you think about the waste tank situation. There are a lot of dump sites available and a new black tank (if necessary) will be cheaper than a composting toilet. There would be a durability advantage of replacing any water supply lines, I used PEX throughout.
Good luck with your project and, if you have the ability, document it so other Forum members can benefit.