Hi Dudley, welcome to FiberglassRV, we're glad you're here.
I've used Marine epoxy successfully in plugging holes in my trailer. It's available just about everywhere, I bought it at a big box store.
I took the spare off the back and plugged the bolt holes, plus took the stinky slinky tube came off the front of the trailer and plugged the rivet holes there. This stuff is mixable white, sandable, paintable and waterproof:
Ten Forward - 2014 Escape 5.0 TA
Double Yolk - 1988 16' Scamp Deluxe