How many Miles per gallon can you get?

I own a used bread van that was converted into a mobile chuck wagon. It is powered by propane gas, but I can never figure out the mileage because it uses propane for the gas stove, oven, hot water and refrigerator. How many miles to the gallon does propane or natural gas get to the gallon compared to the same engine powered by gasoline? When gas was going for over $4 a gallon, LPG was costing about $2.80 a gal. in my area. Also, is there a big difference between LNG and LPG?