My 2018 6.7 Ram with 295/70/18’s gets 11-12 on flat land with my 15.5k bighorn on. In the mountains, I’m in 9-10 range. I did get 20 on a solo trip to LA with a very light foot. Can’t see a dually getting any better with 4:10 gearing. I used a friends Ram DUALLY and it never got over 10-11 pulling.
I also have a 99 Dodge 2500 with a 5.9 and some minor goodies in the engine. 5” exhaust turbo back, DB manifold, 100 HP injectors, fuel map chip and I got, $hit you not, 16 towing my bighorn on the flats. A light foot solo yielded a whopping 26 mpg, sadly, a 21 year old truck is not reliable enough to take it far for my liking without a full resto. hence the 2018 upgrade. Anyway, I can afford the fuel but I am still milage conscious and would love to spend less to do it. It also kills me to pay a $150 power bill too. So, I save where I can even though I don’t have to.