How many of us are there? Owners of Dodge based RV's?

I assume the black water vent exits the top of the tank and does a U-turn downward? If so, and it's exposed to subfreezing air, frost could build up in the vent tube. Enough build-up could block the vent, forcing sewer fumes back into the motorhome interior.

A vertical vent is mostly inside the motorhome, effectively preventing frost build-up.