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 know that I have had great luck using the Gorilla products. Perhaps you may find one of their adhesives that will work the best. The most important thing is that you use something to get inside the gap and clean the surfaces with alcohol first to insure a good bond. Determine what the composition is of the two surfaces and then do a search for the best adhesive.