You are not quite on target with this situation.
The ones that have a fiberglass shell that fully encloses around the underside of the trailer have a subfloor material on the inside of the trailer.
The fiberglass the shells are made from is not thick enough on its own to act as a floor to support the weight
of people walking on it, that is why there would be plywood or some other strong subfloor installed in the interior over the bottom fiberglass of the shell. But not all FGRV trailers have fiberglass underneath, some of them just had plywood that was bonded to the fiberglass at the outside edges of the floor.
If you have a shiny surface under the carpet it might be a layer of vinyl flooring that was installed over the subfloor.
If you want to know if you have a trailer with a shell that has fiberglass on the bottom just stick your head or an inspection mirror under the trailer and look at the bottom side.