It really depends on just what is behind the fiberglass. If it is wood blocking, water infiltration may have caused rot. Anyway, I once repaired a similar problem by cleaning out the holes with an appropriate size drill bit and injecting a marine wood rot repair product into the hole. After allowing time for drying, I solidly packed the holes with Bondo, drilled appropriate sized pilot holes and reinstalled the mounting bracket using new lag bolts.
The repair held up OK but I was always a bit leery of it because I had no way to inspect behind the "fiberglass shell."
Happy Camping! ///// Richard D.
2006 4x4 Ford 250 SD / 2007 Flagstaff 827 FLS
One very patient wife and one furry child who travels with us. Forty-two years of trailering and camping, and I still have a blast.