Caulk on a scamp
belly band is cosmetic only. Does nothing to seal out water. Belly band is just trim to hide a seam that is sealed when the top and bottom shell is fiberglassed together on the inside of the shell.
The only thing that caulk does is keep dirt from collecting in the seam and then running down the side when it rains. All you need is a tiny bead that fills the gap, does not need to go up the side of the camper to "shield" the seam from water.
Acrylic latex caulk available at big box stores works ok. Indoor/outdoor painters caulk is one version. Silicon caulk is evil on fiberglass.
Just a tip (pun intended). Push caulk don't pull it. You use the caulk gun to push a bead not to pull one. This forces caulk into the crevice, pulling just smears it on. The tip with caulk coming out going over the bead acts to smooth the surface. Trick is to only feed caulk at the rate you are moving so that you don't get big glob on tip and having the tip opening just a little bigger than the width you want your seam to be. That and wipe tip often.
Lot of people know this but have seen a fair number of caulk jobs that were done by folks that clearly did not. Such as the former owner that smeared silicon all over my trailer in an attempt to seal it. Removing that is two days of my life I don't get back.
If the aluminum band is sun warmed and too hot the latex painters caulk will set on contact, makes a mess. Needs to be warm enough outside but not hot and the aluminum will be warmer than the air if it is sunny out.