I have a 39' 5er that is just over 14k lbs dry, 2100+ lb pin weight, and GVWR over 16k lbs. When I stayed in-state in Michigan I had no problem pulling it with my '21 2500 HD gasser, even on trips of close to 200 miles. However, when we made plans to start taking it out of state (Georgia, Texas) I decided to move up to a '22 3500 HD dually diesel. Pulling through any kind of mountain elevation, like in West Virginia, I knew I was going to be happy to have the diesel and the extra stability of the dually. Overall I was seeing about 2 mpg better in the diesel than the gas when pulling that kind of load, and about 3-4 mpg better unloaded. Granted I went from a CCSB 2500 to a CCLB DRW 3500, so the truck weighs more and is even less aerodynamic.