Screw holes don't enlarge from the fiberglass being brittle. They enlarge because of movement. A little bit of frictional sawing action done by the screw threads over the years. The shell, the screws and things such as vents do not all move in unison. It is subtle but over time some screw holes will enlarge. Not all of them, it depends on how much flexing the shell is doing and what kind of wind loads while driving are on the objects being held in place.
I found quite a bit of that on my 1980 Sunrader where everything was attached with screws, no rivets were used on that FGRV. They did put some blocking behind some of the screws for them to grip into but being particle board it was only slightly better at holding up to friction than foam would have been. Plus of course where it got wet it lost all its screw holding ability.
There were also screws that went into foam insulation right under the roof surface. I had one primary area for that which was the screws holding on the roof rack. In that situation 75 percent of the screws had lost their grip due to oversized holes and were only being secured in place by all the roofing compound piled over them and the bracket feet of the roof rack extending out onto the roof surface.
In areas such as the baggage door, propane
compartment door and water heater I was able to access those areas from the interior inside of cabinets and under the dinette benches. I tore out the old blocking and replaced it with new. Where there was no blocking for the screws to grab into I added some. Everything resinstalled with new butyl tape and brand new screws too.