Lol, I know this thread is old, but I just have to add for others stumbling through here.
Adding the two heaters is always a good idea, but adding an electric blanket is priceless. I hit temps of -25 below, and have never experienced a cold night. I don't even have a four season 5er. But I have made many modifications to make life easy and comfy.
And aslo some people use very little propane, while others use massive amounts it seems. I have dual 30 gal. Set up and keep 2 spares full to switch out. If you keep your less than 100 gal. or larger tanks empty in very cold weather, they will develop moisture in the tanks, and then in your lines, and make funky noises and problems. My tanks last me about 2 months, and I cook 3 meals a day. I save gas by using a 100 watt incandescent light in my bay where the water line enters. Even at -25 my line won't freeze even if I don't run the furnace.
Just food for thought.