I have repaired two trailers with roof leaks. One at the front and the other in the rear. Rear one was so bad that it damaged the floor and framing under it. My thoughts are, if you see rot on the paneling what is happening behind the paneling? It is quite likely that if it is a wood structure, there is damage to the framing. I have removed the floor sheeting, ceiling paneling and wall paneling to make repairs. I have replaced floor, wall and ceiling framing too It was after paneling was removed that I found all the damaged framing that I had to replace.
I was able to secure replacement paneling for the ceiling and most of the wall from a RV dealer. On the front leak problem, I could not get enough wall paneling, so I used a "Bead Board" paneling for the entire front wall as an accent wall in the bed area. Project turned out quite well.
If you need help on how I did repairs and removed and replaced ceiling paneling send me a PM.