The problem with silicone is that it can't be painted, unless you get the special, hard to find, paintable style. Conventional silicone does not accept paint
Second, once you've used silicone, nothing else will ever stick there unless you use silicone again. The only way to fix that is to grind down into the fiberglass to remove any residue, before trying to use something else.
3M 5200 is designed to adhere to fiberglass, is flexible enough and is paintable. The bond is generally stronger than the fiberglass. Epoxy is the same deal. Very strong bond, paintable and flexible enough.
If you're going to add a wooden "beam" to the overhead, a very good way to add a lot of strength is to add fiberglass cloth over (actually under) the wood and out onto the surrounding fiberglass roof several inches on each side. Add a nice smooth radius on the outer edges of the beam (which might just be a 2X2, or 1X2 piece of wood) and lay the glass with a small brush and epoxy or polyester resin. About three layers would be nice. Each reaching out a bit less on each side to make a nice tapered layer. Then lightly sand and paint
. The beam can be mounted with either 5200 or epoxy in preparation for the overlay of glass cloth. Sticks to the floor will hold everything in place until the glue is set. Then glass it. Use a 1/2" radius router bit to round the edges. This allows the glass to lay smooth over the curved edges and make a kinder place to bump your head than a square corner. With that, you could easily stand on the roof without it collapsing.
Epoxy sticks to polyester fiberglass very well. But polyester doesn't adhere to epoxy well at all. The trailers are made of polyester. So once you start with epoxy, stay with it in future repairs. You can get different formulas of epoxy that set up at different rates. A medium is good for this kind of work in warm weather. Just don't make a large batch and put it in a single cup, or it will get hot and harden too soon. Use a paper plate and get some 1" china bristle brushes at Harbor Freight that you just throw away as you go.