The easiest one to find at an RV dealer is Proflex RV sealant. Dicor lap sealant is for RV roof caulking. The non-sag version mentioned above, it's okay, but it's definitely different from the typical Proflex style of caulk. I have used both and would tend to stick with Proflex for the OP's application. The Dicor is more for a bigger bead, IMO. There are equivalents to Proflex which you can probably buy at a home center store, but I don't know what they are. I believe one of the Sikaflex caulks is just like Proflex, and also there is a Quad brand something or other. Proflex RV is the safe bet, though.