I assume you're removing the hatches and riveting sheet steel over the openings? You want closed-end (which means they won't leak through the mandrel in the middle) stainless steel rivets, either 1/4":
If you got the $70 HF riveter (the pneumatic kind, I assume) then you can do either 1/4" or 3/16". I'd go 3/16" for a patch like this; it's all you really need and they're $0.20 each instead of $0.50 each for the 1/4".
Rivetsonline is where I've gotten all the rivets for my bus so far.
Note: the "grip range" property gives the total thickness of material (all layers combined) that that particular rivet can handle. In the case of a patch like this, you add the thickness of the roof sheet (20 ga. or 3/80", most likely) to the thickness of your patch (say, 16 ga. or 1/16") to get the grip range you need to have covered (3/80 + 1/16 = 0.1). Technically this is outside the grip range of the 1/4" rivets I linked (.125-.25), but perfect for the 3/16" (.020-.125).
Edit: here's a useful thing on blind rivets: https://www.fastenermart.com/underst...nd-rivets.html
To use a 3/16", you want to drill a hole between 0.192" and 0.196", for which a #11 drill bit is ideal. HOWEVER: the rivets you get may actually be pretty far off their nominal dimensions (the 0.25" rivets I got from them were actually 0.26" in diameter, so all the F bits I'd bought drilled holes that the rivets didn't even fit into), so wait until your rivets arrive, measure their diameters with a caliper, and add between 4/1000" and 8/1000" to get a drill bit size that will work for them.
Also: to make your patch watertight, run a bead of seam sealer (like Dynatron-550) around the edge of the underside of the patch before placing it on your roof, and also put a dab of sealer around the hole before you place the rivet and pull it (then wipe off the excess sealer with mineral spirits after you're done).
Also also: clecos will make your riveting life much easier. Here's a good starter kit: https://www.amazon.com/gp/product/B008PJWB2S/