Looks like a sealant in the photo. You should be able to tell - check other areas - a molding should be consistent in size but sealant would be wavy and the thickness would probably vary.
It would need to bond to paint and maybe fiberglass (if your high-top is unpainted). Surfaces would need to be really clean.
A Self-Leveling or Control Flow sealant would give a nice finished appearance and would easy to use. You don't have to spread them.
I've used Dicor Self-Leveling sealant on fiberglass but I'm not sure if it bonds to paint. Maybe call the manufacturer.
https://dicorproducts.com/catalog/roof- ... -sealants/
An RV repair shop should be able to recommend a sealant from the brands the stock