I have no experience or expertise in this issue, but I can tell you a few things.
removers would damage the underlying fiberglass. Some that might not, like the citrus based ones, would be very messy and hard to do inside a trailer, because they usually require washing off with water. Plus there is the issue of fumes, but that might be ovecome by using large box fans.
If I ever tried to coat the inside of my Casita
, I would consider this: I would aggressively scrape away any loose paint
. If there were questionable spots where the paint is not fully adhering to the fiberglass, I would spot-treat them with a paint remover that I had tested to be sure it does not soften or damage the fiberglass. If there are any seams or obviously rough spots, you might feather them in using something like auto body filler (e.g., Bondo).
Then I would consider coating the inside walls/ceiling with bed liner material like that is used in truck beds. I suggest that you could do the many hours of masking any areas that are not to be coated, and save money and insure the job is done the way you want it to be done. However, I am not sure if the commercial bed-liner companies do lighter colors that would be appropriate for your interior. All I have seen offered is black, but you can check with them.
Alternatively, you could apply the bed liner yourself (probably brushing it on). I checked and there are brands of DIY bed liner material that come in all kinds of colors.
My thinking is that if bed liner is suitable for a truck bed, and stands up to the kinds of abuse that it gets in a truck bed, it would be a very good material for your interior. Bed liner is also said to be inuslating with regard to temperature and sound.
Hope this helps,