I can't tell from the pictures where the damage is located. My guess is up near the top where marker lights are and not down by the diamond plate.
I fixed something similar but easier to get at. My TT front looks almost identical, except I have a window. Trailer leaked up at the top seam and it ran down inside and came out at the window. Made it look like the window was leaking. I fixed the leak at the seam with eternabond tape. I took the window out and found the hole was cut too big for window that was installed. The water leakage had rusted staples holding the siding to the wooden window frame. The siding in the front wasn't like the rest of the trailer. The side walls in my trailer are an outer skin of fiberglass bonded onto Luan and that is bonded to a foam core and the inner Luan skin. When I say Luan, I mean a .125 thk 3 ply plywood. The front cap is made up of a wooden frame with the Fiberglass/Luan stapled to it. Meaning no foam core or integral inner Luan. Makes it easy form it to a curve. Any way, I had to attach the fiberglass/Luan back to the wooden frame with staples. I didn't have any wood rot to deal with so it was pretty easy. I ended up fiberglassing some of the perimeter in to make the hole fit the window correctly. Re-installed the window and have been leak free for a few years.
I your case, if I felt I need to fix it, I'd take the seam apart from the outside. There should be a header board the runs from side to side the roof plywood and the fiberglass siding is attached to. As long as the header isn't rotted, you should be able to re-attach the siding along the top edge.
2017 RAM 1500 4x4 5.7 HEMI
2015 PCW ECON 18RBS