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...