All of the tanks on our 2006 Jayco Greyhawk are under the main floor but enclosed in lower compartments. There is a duct from the furnace that feeds air into the compartments. I don't know how effective that is because we haven't done much camping in below freezing conditions. However, for 12 years we stored the MH next to our house (and didn't winterize it) and I do know that temps did fall into the low 20s a few times. With the furnace set at 45 degrees we never experienced any interior water line or tank freezing issues. So I assume that the heat supplied to the tanks is sufficient. I do know that no water lines run through the walls but are all routed internally, e.g. the shower head is mounted on an interior wall. (I always left cabinet doors open to allow circulation of warmer air around the water lines for the kitchen and bathroom sinks.)
