RAM 2500 - Hemi 6.4 vs Cummins - payload vs towing stability

We have a 2012 3500 High Output Diesel 4 X 4. I am getting about 12.5 mpg around town. On our trip from Las Vegas to Michigan I got approximately 11 mpg pulling my 2013 Sundance 26' 5th wheel.

I am not exactly pleased with either of the mileages. I am actually considering trading for a Hemi as the price of gasoline is so much cheaper.

Any thoughts? My wife personally thinks the diesel will have more "get up and go" when going through the mountains.