Haven't found temps yet the furnace won't maintain or run you out. Typical truck camper furnace is 18,000 or 20,000 btu. My 35' FW was only rated for 30,000# btu and heated 3 times the space with ease.
For propane, in temps down to 0*, I can easily make 3 days with a single 20# tank. With temps only down to freezing, can easily make 5 days. Turning the furnace down to 50* at night really helps. Even at 50*, we sometimes have to throw off a blanket, thanks to the oversized 0* sleeping bag we spread out over the top of us.
Cost at local Ace Hardware is about $17. You don't want to go with the ready filled tanks unless it is an emergency as they are usually a bit more expensive but more importantly are not a full tank, sometimes as much as only 15#.
On the rare occasion I'm in a campground with electricity or when on my driveway, I use a 600W fan forced electric heater fed through a baseboard thermostate (designed for the load). Even down to the teens, it cycles on/off holding the camper a nice 76*.
Others have better pics, but to show you my confidence, here is a pic of my daughter and me on a snow run to NC. We drove 300 miles to get into the snow and hoped for a campsite with power, but wasn't worried as was confident we could boondock in a parking lot comfortably enough. Temps were down to about 20* during the night as the snow turned to freezing rain with only a slight breeze. Was surprised to find the outside coated in 1/2" ice though the inside was kept warm at 70*.
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