Pandora's box.
You need to be comfortable setting a limit to how much you repair vs how much you have to repair. HAVE TO being key phrase.
Campers are junk. They are like a child's fort, slapped together with left over wood, stapled, glued and kept dry by using caulk.
The only good ones are either fiberglass or airstream type, not fiberglass panels, but a formed fiberglass shell more like a boat. These tend to not leak.
All others leak. Water is #1 damage and death of rvs.
Scrape off all the old caulk (Lap sealant) on all the edges on roof and sides. Re-apply yearly. This is the biggest part of maintenance. Do not use silicone ever, this hard to scrape off if repairs need to be made, it will be a disaster.
No problem to remove the door and make it a wall. How well you want it to look? You will have to get a new outer panel to match or do something to make it look like it matches. When you open the wall up you will see how it is made and just copy that when building in the new wall piece. Put in a window as desired.
Replacing the floor is going to be involved. You likely will find rot on the lower part of the wall too, and going all the way up to where the leak originated - could be windows, but could be at the roof, or even could be condensation if they used it a lot in cold weather.
If you can rebuild it all from the inside that is great so you dont have to remove any outer panel, but be prepared for major work.
