Aluminum siding on trailers usually starts at the top and goes on each piece under the previous piece. To remove, start at the bottom, after removing all obstructions such as windows or other opening covers assemblies. It could simply be an attachement staple pulling out or potentially water intrusion, but getting the siding removed should allow you to see what the problem really is.
Freelander
Don, Toni & Taren
Wilderness 5'r, F-250 SD
Want'n to be Free'r