Jayco rear corner delamination

You'll need to expose that portion of the trailer that has the damage. I would suggest to remove the closet wall to inspect. Replace the framing in that area and then you'll have to either replace the exterior plywood exterior filon backer or, if still sound, inject adhesive between the backer and the fiberglass. There are a few ways to apply pressure to the filon exterior while the adhesive is setting. If you have a building that can be used as an anchor, wooden braces forced between the building and the trailer is one way.