I think each brand of trailer is built just different enough that the doors are not "filled" the same. Someone has mentioned that a
Scamp door has what they called "buffalo board" in the interior. I know when I pulled the door window out, there were raw edges of something that looked like pressed cardboard around the edge. I sealed that with waterproof glue and clamped the edges. I could see where water had seeped into the area and without the edge being "waterproof" it could wick water. YMMV
Con, has created a 16 page PDF that shows a couple of pictures of a pulled apart
Boler door. If you think it will help, download it here:
Fiberglass RV - Document Center - BolerDoorFix