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Using your propane furnace will provide some hot air to your tanks; However, many campers use electric heaters in their campers in winter to save money on propane use. This, of course, will reduce the amount of warm air to the tanks.
I would guess if it gets really cold, much below O, you would risk your tanks freezing up. If you have a sewer hook up keep them drained out, close the dump valves when really cold and put a little antifreeze in each tank to protect the valves.
2012 Toyota Tundra 5.7 Litre DoubleCab
6 speed, 4.30 Limited Slip, Tow Package
2013 Jayco White Hawk 27DSRL
2010 Jayco JayFlight 26BH sold
Yamaha EF2000is Generators (2)
1996 Honda CB750 Nighthawk