I just traded a 2012 Ram 1500 Outdoorsman 4x4 QC with 3.92 axle and e range tires. The truck averaged 8.64 mpg on an 800+ mile trip to maine and back. I averaged 13 in the winter and 15 in the summer. I didnt travel highway much at all unless I was pulling my 7500lb camper.
I just upgraded to a 2016 Ram 3500 6.7 CCLB SRW for the crew cab and possible upgrade to a fifth wheel in the future. I haven't pulled with it yet but I have been averaging about 2mpg better.
My oil changes are negligible as they will be on the same interval however the cost of fuel filters are an extra 100$ or so.
Having a truck that can haul and tow anything now is awesome, expensive but awesome. This weekend I picked up 50 bags of mulch and 30 bags of top soil in one trip. That's 3.7 cu yards of mulch and 1 cu yrd of soil. Never could have thought about that in my last truck.