Ram 2500 Cummins the most fuel economical

I get pretty good mileage on my 6.7L F250, max is up to 24.5mpg in city/highway combined. I'm pretty certain any of the major brands newer diesels can do just as good. I don't think the brand is what will determine your mileage but instead it is mostly the driver.

You're not going to find anything that has great mileage while towing.