Personally a gas engine is the worst engine for pulling any weight on RV's. Once anyone drive a diesel and feels the pulling power they would never ever go back to gas.
I have driven only diesels for that last 20 years. My current daily driver is a Sprinter. I once overloaded it almost to the danger levels and it climbed the Rockies like it was empty. My wife went out to the parking lot to move it and it was so heavy she thought we lost the transmission because she has to use more pedal to move it. Dodge is coming out with a diesel in this version, I would wait. Some are saying/getting up to 30 mpg in the dodge new diesel.