Fortunately I have not needed to glue the fiberglass on my TC. I would use the blue foam.
This link,
Two Northern Lites with Cab over failing and Basement sides, might offer some help.
I would do a test section using Gorilla glue. It is available at most hardware stores. I would not use silicon.
For locating the leak, my first guess would be the weep holes in the window frames. They get plugged and can let water in the TC. Another place is the seal between the window frame and fiberglass.
Every TC I have owned has had window leaks at some time. I have had to rebuild the wood inside the walls in three TCs due to the window leaks. On my 1988 Bigfoot (see photo), I use butyl tape only (sticky on both sides-see photo), and absolutely no silicone. That is what was used by the manufacture, and did just fine for twenty years. The butyl tape is common at RV Supply stores, and I have heard it is available at Home Depot in the caulk area of the store.
I am having issues posting with photos. I will try separating the photos from the post.