I have only removed the "sponge liner" in a few places for fiberglassing in cleats to mount additional shelves.
But I can assure you it would be a big job. Mine is apparently glued on with either a construction adhesive or a contact cement. Whatever it was, it is really glued down good.
If you were to replace it with something like carpeting, then it would be a lot easier because the residue could be left. But if you wanted to prepare it for painting
, all the residue would need to be removed down to bare fiberglass. Once this was done it would require a lot of filling and sanding.
Years ago we had a Compact Jr. that was painted inside from the factory. The fiberglass lay-up on this trailer was quite nicely done in fiberglass roving. A spackle paint of white with brown flecks was originally applied. I cleaned it up well and applied a new coat of latex enamel. This went on nice and looked great. The problem was sweating due to no insulation.
Concerning the mildew smell -- I would bet the plywood floor is the main culprit. Another prime possibility is the cushions. The mildew that gets on the ceiling is mostly a cosmetic problem. I did as a previous poster, cleaned well, then used a mildew resistant paint as an undercoat, overcoated with a white floor enamel.