The fiberglass fibers are sealed in resin they just provide strength. Like straw in clay bricks.
Insides are almost always covered, need the insulation to prevent condensation and with fiberglass lighted from inside at night would be an interesting magic lantern show of my getting undressed for bed.
Ensolite is a closed cell foam with a tough vinyl surface also known as elephant hide, the foam provides the insulation. Vinyl surface is easy clean.
A form of marine fabric almost like a felt sometimes called rat fur, applied over reflectix bubble and foil insulation. Synthetic material so it won't hold water or stain easily.
Some have an inner and outer shell with insulation in between. Old U-Haul
was one like that I think and there are newer ones too.
I think that a sort of marine or indoor outdoor carpet may also get used but am not 100% sure on that. Recall reading it on one of the posts and sort of recall it from a camper at a rally
Go to the rally
and check them out. Lot of different ways they are done. those inner cabinets and seats etc. of fiberglass add strength, they act as corner braces between floor and wall or at the corners. Some help support the roof.
The cabinets and fittings are a hard gel coat that is smooth and polished on finished side, and are a hard semi-rough surface on the unfinished side. Good side is like fiberglass shower insert, unfinished side is smoother than back of those usually are. You could probably check those inserts out at the local big box home improvement store.
Attachment is by going through outside shell, or for double wall there maybe wood between walls at points where things need to attach. Some models (Scamp is one) use blocks that are fiberglassed like tape or epoxied to the location where a screw is needed. If you have ever used cleats to mount an open backed vanity cabinet it is the same idea.
Hit the rallies, fun day and the people tend to be a lot of fun, even if you just stop out for a day.