I've replaced several rivets in my Casita
. Both ways, with replacement rivets and stainless screws and acorn nut. First thing you will discover is that those things don't go through at 90 degree angles to the inside, so a screw and nut don't align and look crooked and funky. However, the correct rivet bends in the middle to self-align everything. So I use rivets now, not screws.
OK, use a 3/16" drill to drill off the rivet head, then STOP DRILLING. Take a straight punch and hammer to tap the rivet out, to the inside. Go inside and recover the rivet and acorn nut. Use pliers, vice, or whatever, to grab the rivet shank (BTW, notice how it's bent?). Unscrew the nut off the rivet, and re-use the nut.
Place a new snap-cap base on a new rivet, apply sealer, poke it through the hole, and have a helper inside the trailer place the acorn nut over it, and hold it snugly against the fiberglass surface, while you set the rivet at the outside. That pulls everything tightly together.
used silicone sealant, didn't they? Just sayin'