I have 2 40# tanks one behind the other. These are 75# when full and a real pain to remove the rear tank and replace it full. I was told best to keep rear tank closed and just run off the front one. As full-time travelers we avoid cold weather but the only in South Florida have we totally escaped it.
We survive 40 degree and above with the fireplace/heater and heat pump. If the weather man warns of freezing temps I'll give the front tank a tap test if it has been a while since it was filled. If it sounds like it may be low I'll open the valve to the rear tank. If the turnover shows red in the morning I'll get the tank filled, if not I'll turn the rear tank off. If the conditions remain the same for couple nights I repeat the same routine till front tank is empty or the weather moderates.