Fiberglass RV's will generally be aluminum rivets. Steel take a lot more effort to install since steel is harder to bend than aluminum.
The rivets sizes vary with a majority being one or two diameters and lengths. Steel rivets can be more common in hardware and big box stores so if you buy them locally need to watch and make sure you get aluminum.
Here is a link to an animation of a blind rivet being installed
Just wait a few and the image goes through the process as a mini movie.
Scrap aluminum flashing or scrap gutter material is good practice stock. Drill holes, insert rivet, clinch with rivet "gun".
You can even drill a hole just a touch bigger than the rivet diameter in wood and practice "setting" or "clinching" a rivet that way. You can also remove the heads but the unless the wood is thinner than the rivet is long so the hole you drill goes all the way through you won't be able to punch the rivet out.
Good quality rivet tool saves aggravation. But the punches you can get a set of cheap ones at harbor freight. Just one caution about punches (and drill bits) they are hard steel, they will snap rather than bend so keep the force straight on them. Do not pry.
One other tip I learned from a member here (Hi Carol!) where you need an "in between size". Just get one longer and cut it down with the shaft out. You can remove the center shaft of rivet by tapping it out, cut the rivet tube a little shorter with a hacksaw then put the shaft back in and use it.
Fiberglass is not all going to be exactly the same thickness so on a curve or in a thick spot the regular length you have been using may be a bit short so you have to use a longer one cut down. Scamp
13 closet, rivet on the curve of the roof is one I ran into. Or if a hole gets oversized and have to use a bigger rivet.
Reminds me; rivet head may want to spin when you drill, if that happens stop. The spinning rivet will enlarge the hole it goes into. Gently slip something like a thin bladed screwdriver under the rivet head and gently pry up to put tension on it while you drill, this prevents it from spinning. You may want to put something on the fiberglass next to the rivet head so you are not prying on the fiberglass. You are NOT prying the rivet out, just pulling it up enough that the base wedges in the hole so it stops spinning and lets the drill remove metal. Might still slip and spin a bit as you do this, if it does just get a new grip with the pry and drill some more.
It doesn't take much for a steel drill to cut through an aluminum rivet. But the cutting edge of the drill can sort of grab the rivet. If the center shaft falls out and the rivet tube and head is still in there, you can go a drill bit larger than the shaft and very gently use it to cut a hole through the head which will cut the top of the tube part from the head and the head will come off, possibly spun up on your drill bit.
YouTube pop rivets or blind rivets tutorial and you should be all set. Then search for remove the pop rivet or blind rivet tutorials :-) Really easy after you do 3 you will be an expert.