Butyl tape is user-friendly and lasts for decades, so it's a good choice. Other products may work also.
The main thing is to get the vent off, clean and prep the mating surfaces (roof and vent flange), and then put whichever sealant you choose under
the flange. In other words, if you are using butyl tape, you would apply it to the solvent-washed underside of the vent flange, and then re-insert the vent into the camper roof. Then you can either re-rivet or use other fasteners to affix the vent to the roof.
There's no need to apply sealant to the outside or top of the flange (although going by many of the campers I looked at, it must be very tempting*).
You are right on to stay away from Silicone. It's the gift that keeps on taking.
* I suspect that the layers and beads of sealant on the outside and on top of many vent flanges and around the edges of windows
are probably attempts to fix leaks
the "easy" way. Unfortunately, the leaks
usually persist (or come back) until the item is re-sealed properly (removed and re-sealed from under the flange).