Not knowing the thickness of the fiberglass cap, I might try sheet metal or wood screws with a careful consideration when drilling a pilot hole to ensure the hole is large enough to prevent cracking but not so large that the screws will not hold. I would also use epoxy like a thread locker. Another choice, if you can get inside, would be to use a small diameter pop rivet with a pop rivet washer on the inside.
For the sidewall, I would probably just use the screw and epoxy method.
My end caps are so thin I used the anchor type that have metal wings (?) that expand and grip the fiberglass from behind. There was no way to get a renforcement washer behind the cap to use pop rivets wood screws are tapered and would not hold for long and the fiberglass to thin for metal screws. No problem with water leaking around the screw holes, my end cap extends about a foot from the trailer structure and is open at the bottom, any water would just drain down the fiberglass and out the bottom. I did seal the mount to be sure.
And remember to use caulking to seal against water intrusion.
I'm planning on installing a rear & side view camera also. I passed a travel trailer a while back; and the camera was mounted to the rear bumper on top of a small 3' tall post.
I'm thinking of placing the rear camera to the ladder rail. Another thought is to mount it inside the trailer, looking out the rear window. Don't know if this will work.
My plans are to mount the side view camera on the bottom of the front awning arm.
Couple options as stated earlier. If it will be permanent, I might go with aluminum pop rivets. If you want it to be removable, stainless steel or aluminum screws. Can use rubber grommets or rtv to seal holes under the screws and rivets.
'07 Everest 344J
Will Pops (not for long)
'11 F-350 DRW CC LB FX4 6.7L(Daily driver)
Rugged Trail BFG LT245/75 R17 LRE
Husky 16K Dual Pivot, Blue Ox Bed Saver
Garmin 465T GPS (Trucking)
Dual Honda EU2000s
Use short stainless self tap screws, but predrill the holes. If the cap is thin, 1/8" or so, get a tube of silicone or 3M-5200 adhesive/sealant. A little bit around the screw holes to seal them will also bond the bracket. Use a little denatured alcohol to clean up with.
'06 Ram with a Cummins
'09 36MAX1 Carrilite
Me, Alie, and Salie
If your sure about where the brackets are going and really don't want them to eve fall off ues sheet metal screws and bed the brackets with Dupont 5200 adhesive/caulking but be advised once it dries its permanent!!!
07' Dodge 3500 6 speed Cummins Diesel Dually/6.7L Bully-Chipped /
Exhst Brake/07' Cedar Creek 37CDTSD Daydreamer fiver
Mallard @ Frau Blücher
Now lets Bow your heads for the men's prayer.
I am a man, but I can change.
If I have to, I guess...
I like the idea of mounting a camera inside a rear window; no new leak risk.
That said, on our sailboat, we drill pilot holes, then through bolt or run sheet metal screws in and out to cut a hole, then mount with adhesive caulk, the screws holding till the caulk sets. Getting anything loose again involves chisels and grinding, then epoxy paint to cover the scar.
The boat has been in the water, exposed to the weather, since 1981. No leaks.