I would not rely upon the strength of the fiberglass. Instead, through-bolt to a metal backing plate the same thickness and dimensions as the mounting flange. It's okay if the backing plate is larger, but not smaller, but make sure all corners are rounded. Waterproof all under the mounting flanges with polysulfide or similar.
Install the mounting flanges as close to an interior wall as you can get, to provide maximum stiffness. You do not want the weight carried in the middle of a large open space, which will cause the fiberglass to flex under load every time you hit a bump and which will eventually cause the fiberglass gelcoat to crack.
Very important, if the fiberglass has a balsa core (not uncommon in boats, but I don't know about RVs) you will need to fill the bolt holes completely with epoxy resin (basically epoxy the bolts into place) to ensure that any leaks that DO develop will not soak into the balsa and cause it to swell or rot.
Engineering: Finding complex solutions to simple problems you didn't even know you had.