The product you need is Dicor , self leveling sealant . Comes in a tube for a pump gun. Available at all RV parts suppliers.
Clean the existing sealant surface with mineral spirits, and apply the new sealant, over the crack or completely around the vent.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.