[JRTJH]

Let's "guess" that you have a 30,000 BTU furnace. That is the INPUT BTU, not the actual heat capacity BTU, a large part of that heat goes out the side of the RV as furnace exhaust. LP gas has about 91,600 BTU in each gallon, so you "should" be able to run your furnace for 3 hours on a gallon of LP. That means you "should" get about 21 to 24 hours of "CONTINUOUS" furnace operation from a 7.5 gallon LP tank.

Realize that your furnace does not operate 100% of the time and that when it is operating, during a part of that run time you won't be getting "hot air" into the RV (when the furnace is heating up it blows cold air).

There really is no way to determine (best guess) how long you can stay comfortable inside your RV before you use all the propane in your tanks. The outside temp, wind, where you're parked (sun or shade), direction the wind is blowing, how warm you like the inside, air leakage from roof vents/windows, how much "other propane you use" with the stove, oven, refrigerator, hot water heater, etc.

Best guess would be that most of us can get anywhere from 2 to 7 days from each 7.5 gallon tank depending on the above factors.