My 2013 RAM/Cummins - worst mileage was 14 mpg. That was loaded with a Fourwheeler type camper, pulling a 7x16, 5000 lb load, enclosed trailer and running into a 20 mph head wind at 65-75 mph. Generally, it got 16-17 towing and 18.5-21 empty.
I'm currently buying my 5th RAM/Cummins, but a 3500 Cab n Chassis this time. Getting the optional fuel tank at 75 gallons. So, about 1200 or more miles of range. That makes buying Diesel much more economical since you can shop. Recent trip, $3.40 in UT, $4.28 in OR, $5.60-$7.90 in CA.
The extra maintenance for Diesel is BS (IMHO). More fuel filter changes and an EGR filter every 70k is most of the diff. The new (12 yo) emission equipment is solid on the Cummins.
Compare driving a Gas vs. Diesel in the mountains and max GVWR. There is no comparison. The Gas will wear you out as you slowly climb up at max RPM's only then having to drag the brakes down the other side. Diesel is calm by comparison. Good representative videos on Fast Lane Truck.