dwenze12, you don't say what type of motorhome you have but our mirrors are installed the way you describe. The bolts go through the fiberglass and have large washers with washer locks to keep the back of the bolt on secure. I can access through the front generator area.
I've had to tighten the bolts a couple of times as vibrations had loosened them over the years.
Good luck,
Mark
Mr.Mark
2021.5 Pleasure Way Plateau FL Class-B on the Sprinter Chassis
2018 Mini Cooper Hardtop Coupe, 2 dr., 6-speed manual
(SOLD) 2015 Prevost Liberty Coach, 45 ft, 500 hp Volvo
(SOLD) 2008 Monaco Dynasty, 42 ft, 425 hp Cummins