A good set of pictures of your new treasure, inside and out, would help a lot with recommendations.
I'm assuming the insulation is Ensolite, a spongy foam with a textured vinyl surface… A lot can be done to save it, and replacing it with something else is a big job. Since it is closed cell foam, the mildew is only on the surface, so it can be cleaned up. Loose sections can be reglued, gaps can be filled with latex caulk and textured to match, and the whole thing can be painted. I'd recommend at least trying to save it before you tear it out. The insulation is not structural, but it is important for controlling temperature, noise, light
, and condensation. Bare fiberglass walls you do not want!
can usually be removed, cleaned up, and reinstalled with new butyl and be as good as new. The only one(s) that sometimes need to be replaced are the fixed plexiglass window(s) in the front and (in some eggs) the back.
Before anything else, make sure your running gear is up to par (tires, bearings, axle
, frame, and lights).
Best wishes with your new project!