I work in Yellowstone from mid-May to the end of September. Early and late in the season it isn't unusual to have the temp drop into the teens at night and have a high in the 40's mid-afternoon.
So, unhook the water line and use your fresh water tank. Set the furnace to 60 or 65, run a space heater. Most space heaters are 1500
watts so, with temps in the teens, the furnace will still cycle on a few times overnight. This will keep the tanks from freezing.
On my rig, the furnace thermostat is directly above the head of the bed so about 1/2 an hour before we have to get up we run the thermostat up to 70 or so.
All is good.