I average about 8.5mpg when towing my 24ft, 4200# loaded Travel Trailer. Averaging about 65mph.

I try to use E85 anytime I haul/tow because it's much cheaper and the mileage is the same as regular gas.

 

I actually noticed that not running in tow/haul kept it a little cooler on average. It would still spike to 240, but went down a little more on the flats. It did seem to do better in tow/haul though just for shifting.

 

It's not a Dodge and/or Hemi thing. All gas trucks tend to tow flat-faced trailers at around 7-9MPG. If anyone tells you they are getting over 12MPG they are either lying or going downhill both ways (as they wave at me walking uphill to school). My 2003 Ram Hemi with 3:55 gears got around 7-8MPG. My 2010 with 3:92s gets around 8-11MPG average depending on the wind. I attest that to the better shifting, tow/haul mode, and better gear selection. My 2003 only had an overdrive button. Wind resistance increases exponentially with speed. At 75 you will most-likely have double the wind resistance than at 55-60.