All depends on how you use it. The tanks on all but the 2250 are heated and enclosed. Which means as long as the furnace is running you are pushing furnace air into the holding tank area. That includes valves, fresh water, black, gray tanks, water filter and water pump.
There is a owner on there that got caught in the mountains. Dropped into the 20's at night, back up to the 30's during the day and his tanks never dropped below 48. I have ben skiing in a 2650.
Now if it is there for storage...that is another question. You would be better off blowing it out or as others have said a space heater of some sort with a t-stat. However, the small water lines that run to the fixtures need protected as well.
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