My 1st camper was a 15 foot Prowler that had water damage to the roof near the antenna and one of the back corners and back window area had water damage to walls.
Cut out all of the ceiling paneling, the whole back wall, and a few feet up the side wall from the corner.
Replaced any rotted framing lumber from the inside, Lowes and Home depot have a decent selection of metal joining plates that came in handy.
Replaced insulation and paneling and used it for a few years before we outgrew it and moved up to a bigger trailer.
Mine had a metal roof as well, you really have to keep after the seams and caulking.
I would use butyl tape under the vents then go over with dicor self leveling caulk (go over the screws also). Geocel caulk works well where you dont need self leveling like around windows and lights.
I would use Eternabond tape on any seams in the roofing, like where the front or rear cap meet the roof metal, of where pieces of roofing are seamed togeather.
I found a nice product called Revolution Ply that is a thin plywood luan paneling, it comes primed on one side and is easy to work with and inexpensive.
Good luck with your project! It can be done!