Having spent my life in the Marine Industry I know a thing or two about
fiberglass construction. This is a common problem for so many items to be mounted safely and secure in a boat as well as a RV. The manufacturer has a set of plans to go to all the items so they can instal a backing plate. We can build them in place during lamination. Now to do it on the side of the trailer I would look for the vertical side supports .Then you want to see where the brackets would mount. Then you need to go inside and see if you will be able to mount a backing plate to match ones on outside. Say all’s ok I would make my plates at least 2” wide X 1/4” thick X the length Aluminum between top and bottom mounting holes .
You can pre drill your holes in the brackets 1/4”on inside and on the 2 outside brackets you can drill & Tap them (1/4-20 threads )have someone to help hold the bracket on outside level and mark holes . Now drill out your holes use sealant on all the holes and bolts. Run the bolts from the inside out so they look clean and you can use locknuts on outside. NOW STOP JUST AS YOUR BOLTS ARE GETTING SNUG. Walk away until a day or so and now finish up tightening up the bolts.
What this does is seal the holes and when there tightened it won’t leak and the brackets will spread the load out on the side. I would make some type nosecone so to make it more aerodynamic and less wind resistance. There are a lot of different type sealants out there . The best one to use to permanently attach anything to
fiberglass is 3 M’s
5200 . Next best is 4200 it sealed but can be taken apart with some work. Nest is what I use when putting in screws is clear silicone ( I like the toothpaste tube)
Another tip is to remember when you drill into fiberglass that has gelcoat, start the drill in reverse and start to drill a hole. Stop put in forward and drill your hole. This will almost eliminate spider cracking or chipping around your hole.
Hope this helps .