my bet is you just ran the tank empty...pull it and shake it. The propane may outrun the frost line on the tank depending on the temps. There's several factors in how long that tank will last.....outdoor temps and t-stat setting being the biggest factors. Also if fridge or water heater are running and on propane will factor in too. A bump in the t-stat from say 60-70 is a considerable difference in how much propane you're going to use. I've also noticed a huge difference between camping with lows in the 30's versus lows in the 20's. That furnace will cycle a ton more.