What always amazes me is 'how good they look on the outside and how hosed they are on the inside'.
I always maintain that any used camper, when the owner wasn't taking care of the seals and caulking, will be rotting under the skin.... and that includes letting the camper sit outside year around.
Why I believe in a blower door test or take it to CW and have them pressure test it and check it for leaks.... at least ever 2 years and go over the caulk and seals every year, better yet every 6 months.
It's a job but I bet it will be nice when finished.
I'd loose the fiberglass bats and replace that stuff (which retains moisture) with foam sheets. Might as well while you have it tore apart.
Thumbs up to you...