"rocmoc"....Why don't you wait for the new mirror assembly to arrive and then you can see how it comes apart.
Typically, they use long hex head wood screws to secure the old mirror base. They back the fiberglass with a block of wood and thread the screws through the fiberglass into the wood. On an older coach, they might not tighten up when reinserted. You might want to add something to make the old screws grip when you reinstall, like fiberglass resin, epoxy or some other adhesive.
Don & Mary
2019 Newmar Dutch Star 4018 - All Electric
2019 Ford Raptor Crew Cab