I tow a 31ft TT that weighs in around 6500lbs loaded. I don't tow often. About 12-15 times a year. Usually less than 50 miles any single direction. My truck is a 2011 QC 1500 4x4 Hemi w/ 3.92 and the heavy tow package. Towing I get 8-11 MPG. I use a 10K rated Equal-i-zer load level hitch. My truck has no issues towing at all.
I've found I don't need air bag additions. However, I can see where some might want them. I believe the combination of a well balanced trailer and the overly heavy duty load balance hitch likely makes the need for air bags a non-issue.
My previous truck was a RAM 2005 QC 4x4 2500 Hemi w/ 4.10. Granted, this was a previous generation Hemi. I found the engine "wanting" more power on the hills. MPG was 8-10 towing. In other words, about the same as the 1500. This biggest differences: The 2500 was an HD model and heavily over sprung for the size of my TT. Even with a lowered hitch I never could achieve a level tow. The 1500 actually has a better tow height, obvious more engine power, and provides a better overall towing experience
This said, given today's Hemi 5.7 or the 6.4, if I were to do more towing I would opt for the newer 2500. It's simply engineered to handle larger loads over longer periods of time. While the 5.7 is up to the task, the 6.4 is designed as a "truck" engine. where the 5.7 is not. IMHO, I believe in the 2500 the MPG would be the same for either engine. My 2500 got about 16 MPG highway and 13-14 in town. Again, about the same as you would expect from a 6.4