We have forced air from the furnace ducted to our holding tanks. We have traveled in 10 degree weather, and camped in the low teens with never a problem. I do keep a remote temp sensor in the holding tank bay just in case. I do not recall ever seeing it below 40 degrees.
One of the reasons we purchased our Alpine was because of the holding tank and fresh H2O capacity.
Good luck in your search. All of us will be happy to help with your questions
Tom, Patty, Hannah "The Big Dog" and Abby Kat, Indianapolis, Indiana 2000 36' FDS 72232 Our Photos
We live out in our old van. Travel all across this land. Drive until the city lights dissolve into a country sky, me and you - hand in hand.