Here is what we do during December (central Illinois) for temps that sometimes get down to zero, with windshields falling below that.
We have a heated water hose. After dumping the black tank I put a gallon of RV pink in the black tank, then every day I add another gallon. After 3 or 4 days I dump the tank and start all over. Grays stay open. At night when we go to bed I leave a very small stream of HOT water going in one sink.
Even though I have a furnace register in the basement I keep a ceramic heater going down there 24/7.
We allow the furnace to run during the day and night off and on. We supplement with two more ceramic heaters, but we also let the furnace do part of the heating because on our make/model that also puts heat into the underbelly.