Hi Restrac and Brian,
Too many variables to know for sure. How many boats are you pushing out? What type and configuration of the lash-up? What are the wind conditions? What's the lake level? How fast are you going to try to go? What size motor will you have?
My personal data here is pretty narrow-range. Over the years, I have pushed a lot of strapped-up rafts on the bottom end at Westwater. The one time I did a calculation on it, I figured I was getting about 25 miles per gallon -- often with as many as 3-4 other rafts lashed to my cat. Down at Cat, I once pushed a single 20' Jack's cat from just below the last rapid (#28? #29? at the time) all the way to Hite with a Honda five, and not much wind. Cost me about a gallon and a half of fuel, and that was just one boat -- albeit a big one with a smallish motor.
My personal comfort level to get off Cat would be pretty high if I had a five horse gas four stroke and a three gallon fuel tank. So if you consider that propane is maybe 20-30% less efficient than gas, you're crudely in the range of five gallons of propane to have a safe margin -- again with a lot of recalibration for all the factors I mentioned up above.
That hardly qualifies as much more than a scientific wild-ass guess. But hopefully the company could provide a prospective customer with some fuel consumption data to start making a real-world estimate.