MPG for class B models

We have a Pleasure Way 2010 Excel TS model (V-8 and gasoline) and we average a full 16 mph on most trips. We also owned a 31 ft. Jayco Granite Ridge SS, which we loved, but didn't find ourselves using as much anymore, we were more "travelers" than campers, so we purchased the PW. We are very happy with the model's layout, and it is the wider body, as opposed to the PW Lexor. Those few inches seem to make a very comfortable difference in passing one another in the aisle. The transition from a 31 ft. C to a 20 ft. B is "exciting" at the very least, but once you realize how much you DON'T need to bring with you on trips, it makes the actual trip much more enjoyable. The Pleasure Way also makes the Plateau model, which is a Mercedes engine, and I know they certainly get more mpg than the gas, but I can't speak to the exact amount. PW also makes a gas model B+, or small C, 22 ft. which they introduced over a year ago, which has a full stand up shower. I think the Ford-350 van is being phased out very soon though.