I don't know about a propane configured rig, but mine is a gas powered, 5-speed (engine size, transmission & differential still to be determined) that I purchased in North Oxford, Mass. & drove to Havelock, NC. I averaged 6 miles to the gallon for the whole trip. In my excitement of purchase & desire to hit the road to bring it home, I checked all the fluids, gages, steering & brakes, but did not remember to check the tire pressure!!! A few days after I got it back, it occured to me about the tires and they were indeed low! I am confident that my mileage could have been better. Also along the way, I experimented with holding different highway speeds (it'll do 75 on the straight & level) & drove tankfuls at 65, 60 & 55, and found my best mileage occurs between 55 & 60. Once I get it back on the road again I plan on checking mileage with tires properly inflated. Hope this helps!
~(G)Q Arduously Avoiding Assimilation