Something doesn't sound right about the 6.7's. My 5.9 will get 14mpg empty and 12.5 w/ 10,000lbs behind the truck.

 

I thought I'd chime in on the MPG I'm getting on my vehicle while towing, since the weight of the trailer is fairly close. I tow a 2004 Keystone Hornet 27B travel trailer which has a shipping weight of about 4960lbs. I tow it through the mountain passes of Colorado quite a bit and typically get 11-12 MPG going about 65-75MPH. This is hand calculated. My truck is listed in my sig.