I had a 2010 Ram Cummins diesel and now have a 2018 also Ram Cummins. I love the power. Either truck would easily pull the 13000# load up a 8 or 10% grade at highway speeds. Of course the tranny would drop down from 6th speed overdrive to 5th gear but the rpms would still be pretty low. The newer Cummins does need DEF but that is not a big deal. It is getting over 1000 miles per gallon of DEF and the tank holds 5 gallons. I can see no issue with the DEF and emission controls. The newer engine seems to have every bit as much power. In addition the mileage is noticeably better, at least 15-20% better. I recently drove from NY to the Rocky Mountains and got 15 mpg at about 65 mph highway speeds. The older truck would have gotten 13 mpg at best.
In addition to the power and better mileage, there is another issue which is important to me. A few weeks ago I dropped down the mountain side into East Glacier. I don't think I even touched my brakes. The diesel brake did all the work. I would not have even thought about the hill except someone at the gas station was complaining about the steepness and having brakes that were smoking. My 2010 went 80000 miles always with the camper and with a lot of mountain driving and the brake pads still were less than half worn.
I can see plenty of arguments for a gasser. I would have one if I did a lot of relatively short drives. If I drove a lot without the camper. If I rarely drove in mountains.
Of course initial cost is a major downside. I was shocked at the resale value of the 2010. It appears you are likely to get back much of the initial cost in resale.
