Water that leaks
in tries to find a way down and out so around the edges are the prime locations. Or around the door. Look for the signs of darkened wood mention in prior post.
Another indication can be where the sides of trailer meet the floor the fiberglass is attached to the floor, water softened wood can allow that "tape" of fiberglass to pull off of the floor.
If the door looks crooked in the door opening check the wood on either side of the door with extra attention. In a scamp
13 that would be under the couch and in the closet.
Frames that are rusted through or deeply pitted by rust in an area are on the way to needing a repair. Surface rust that does not deeply pit the metal is no big deal. You may want to wire brush/ sand and paint
it or not. If it has been painted look close, painting
can hide pitted metal but it sure does not fix it.
Metal gets more brittle where it is welded look for where metal frame pieces are welded together and check for cracks right along the weld.
It's really not that hard to patch a small area of damaged wood say something you could cover with a paper back book. But is it still leaking water? That becomes another repair.
If the windows
, vents, or belly band is covered with caulk on a 25 year old trailer I would guess it leaked at some point, that caulk is not stock on the old trailers so it was added for a reason if it is there.
Good luck with your trailer and happy camping.