Our (new to us) motorhome has a small section of outside trim that has separated from the fiberglass wall. It's about a 20 inch gap (about 3/8") that I can pull open, but not able to get inside for a ...
I went with the Gorilla two part epoxy. I mixed it in a large syringe from Walmart and applied under the trim. I cleaned under the trim with rubbing alcohol before applying.
I positioned a board with a little force on top of the trim. Looks like this will work just fine.
Will remove board tomorrow.
Note: If you want to use this method, be ready to apply the epoxy ASAP, as it sets up in the syringe in about five minutes.