I'm thinking you're asking two questions here, right?
1) Sealing windows
2) Wood trim on the inside (which probably accepts the fasteners)
For sealing (assuming a flanged window), I would choose butyl tape as my first choice. You could also use butyl caulk or a polyurethane caulk if you didn't like the tape for some reason. You put it on the back side of the flange before you put the window back on the camper.
For wood trim/backing plate on the inside, I would use real wood. I think there are a number of types that would be appropriate, or you could use a decent plywood. I can't see an advantage to using fake wood. It may be less prone to rot, but if you have enough water inside the camper to rot wooden pieces, you have more problems than that. I don't know how well the fake stuff takes fasteners; I would think it might be subject to "creep" under load, but not sure that would be an issue. I haven't researched it because I know I would use wood.
I don't know if the fake wood would be more prone to condensation; real wood does pretty well.
There is also the possibility of coating wood in epoxy resin, but again, I just don't think you should see that much moisture inside your camper.