In my rig, the freshwater tank is under one of the beds. The furnace ductwork runs under that bed and out a vent there. There is no direct venting into that space under the bed, but I would bet the residual heat from the duct work at least keeps the area above freezing. Not to mention that the entire compartment is basically inside the living quarters of the RV (though it is under a mattress and some wood). I don't think our fresh water tank would ever get cold enough to freeze as long as the inside of the RV was kept warm enough for people.
Where is your fresh water tank located?
Steve