On a fiberglass camper, I would fill in with fiberglass reinforcement* and epoxy resin. You can use something like a waxed board on one side to keep things smooth. Make the fill-in oversized, then cut the hole to the proper size (vs. trying to make it just the right size in the first place).
Once you have that done you can paint
the bit of fiberglass that's showing on the outside. Or if you want to get really fancy, fill in both top and bottom a bit and then the flanges will probably cover it (I would only fill in one place though - would decide which one by checking out the inside and outside to see where things would end up in terms of trim, etc.)
I think there are a number of posts in the archives talking about how to do fiberglass lay-ups, so I won't re-hash that here at this point. (Although I do not seem to be able to use the search function here very well.) Of course ask if you have specific questions.
*Something like a biaxmat would be really strong, but a woven cloth would probably be adequate.