My 2020 Diesel 3500HD Single rear get 9-11 MPG pulling my 43ft 16K 5th wheel ( Anderson hitch) up and down the hills of I65 near between Huntsville and Birmingham, AL. Great truck, towed on 3 trips with another this coming weekend. It's much better than my 2013 RAM diesel 3500 did. I think the 8 ft. bed has a lot to do with it. My RAM had the short bed.
Edit: It is my daily driver and I get about 16MPG