Cummins seem to get the best MPG of the big 3 (GM, Ford, Ram). Most folks who have them, love them. Changing fuel filter can be cumbersome, but doable. IMO and vantage point of a semi driver, most RV transport guys run Rams w/ Cummins motors. Of their 2 tranny's, the Aisin is their top of the line. If you end up w/ a SRW, may want to consider Timbrens, airbags or "X" to help the rear end from squatting w/ camper on. Like what a couple others said about DEF, can get at most truck stops in truck lanes OR at any auto store. Usage should be 10 gallons/year as one said. Buy fuel at truck stop if you can due to high turnover of fuel. If not, find a large local place where you see a lot of companies fueling up (utility, landscapers, town/hiway dept., school buses, etc). Others with Cummins can chime in on fuel additives.
A lot to consider, but in the long run, a diesel is by far the best way to go if towing a lot or heavy and still get decent mpg's. Gassers have come a long way and for lots of folks, will work best for their set ups and don't want to deal w/ diesels and worrying about fuel and all. Both gas & diesel have their pluses and minuses. Do not be afraid to ask questions here about them as there is a HUGE amount of folks ready to assist you to answer them. May get some biased opinions and keyboard fighting, but it's all good. Let us know how you make out.