For the roof cabinet rivets you'll be looking at 3/16" diameter Aluminum pop rivets, (not steel rivets') in either 3/4" length or up to as much as 1" length depending on the specific location you need to join components. I would suggest "sounding" the depth to see how much "shank" you'll need for the specific rivet(s), and add the necessary deformation of the bottom bulb to secure it when squeezed because not all of them will require the same depth of rivet lengths. These trailers are not created with the same dimensional characteristics throughout, even within a given length of the trailer's shell and cabinets. The length of the rivets you'll need will vary depending on what rivet you want to replace. This holds true for most of them throughout the trailer.
When you know what depth you need in order to join your components, here's a site that can provide those longer rivets that you won't find in the "big box" stores. (Most big box stores don't carry anything longer than 1/2" in length.) Also, if you have an RV repair place nearby, they will have bins full of them since they use them all the time, and they'll sell you a handful of them pretty cheap.
Pop Blind Rivets | Threaded Inserts & Nuts | Blind Rivet Supply
And you can also find the "Snap-Caps" and matching bases at many places. They are very common and are usually available next to the rivet section. If that doesn't work, then you can get some at:
When you get ready to insert your chosen rivet, (and the Snap Cap's Base Cup,) be sure to put a small bead of sealant under the snap cap base right in the rivet hole in order to make it waterproof when you squeeze the new rivet. If you don't seal the hole, I guarantee it will leak.
And FWIW, those decorative 1/4" X 20 Acorn Nuts that you see attached to the bottom of the rivets on the expanded bulb end of those 3/16" rivets inside the trailer, just be advised that they are not absolutely necessary, and are just decorative, to hide the ugly squeezed end of the rivet. They add absolutely no structural strength to the rivet. They are merely for aesthetics.
If you do want to add the decorative Acorn nuts, obviously it will require two people, (one outside to squeeze the rivet gun and one inside to hold the Acorn nut up tightly on the rivet while it is being squeezed.) Do not attempt to screw the Acorn nut onto the bulb of the rivet after it has been compressed, because if you do, you'll probably wind up spinning the rivet itself, and when you do, you have just destroyed the bond of the sealant to the rivet and, again, it will leak.