You can rivet a steel (or AL) patch through the fiberglass. Maybe one with the standard 14" sq (14-3/16") hole, so you can swap out any rv accessory you choose.
Use a backer strip or washers on the inside so the fiberglass is squeezed between. Optionally, you may use epoxy on each rivet as you set them. Same on the backer strip.
A wide overlap will add rigidity to the assembly.
Many light aircraft manufacturers rivet fiberglass cowling nose bowls, fiberglass landing light mounts to AL, piano hinges to fiberglass... often using rivets and washers or backer plate.