Most likely an epoxy of some kind. I used an industrial sized tube of JB Weld to fix a loose panel on mine, but Devcon or Scotch-weld should work well too. Grainger carries larger sizes than typically found at Walmart or wherever.
Epoxies will bond fiberglass to metal better than most other adhesives.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL