Are you not aware that if you beat water hard enough it boils? All a matter of the energy transferred.
I hope that you will find an answer to this question and share it. I have one propane
genset, but if is supplied by a much larger bottle, and I manage one genset that runs off 10 lb bottles. Even a rough estimate would help.
OK. Here's a way to get to an estimate. The energy contained in a pound of propane (or LPG) and in a pound of gasoline are about the same. Under standard conditions a gallon of propane weighs about 4.2 pounds while a gallon of gasoline is about 5.7 pounds. That's a 27% difference, and low and behold, propane is 27% less efficient per gallon. So, expect to get the mileage from 5.7 pounds of propane that you would get from one gallon of gasoline. Now go get an empirical figure and see if it matches.