Ray, are you certain where the leak is coming from? It's usually from either the windows in the high top or one of the through-hulls on the roof; the plumbing vents, that sort of thing. And no, do not EVER use off-the-shelf silicone sealant on fiberglass, even though it's widely used by manufacturers because it's cheap and looks nice... for a while. But it won't seal well, will leak again, and is impossible to get cleaned off enough for a proper sealant to stick.
Use the proper type of Dicor RV sealant.
depending on whether you need it to be self-leveling or lay a caulk bead.