15.6 MPG towing from Chicago to L.A. with a 21 ft 3800 pound travel trailer in December, on winter diesel fuel, which is lousy for getting good MPG's.
Your TT is the problem. This is the same mpg I get pulling a 35' 10,000-lb TT with an '04 CTD. But the TT has an aero shape and aircraft-type construction. Put a boat-tail on that TT and the numbers will come up. And, "winter fuel" isn't the problem (energy change of 2%), it's the failure to maintain operating temps, quick warm-up and all of this compounded by extra idling.
Yes, the new DODGE diesel .5T ought to have better aero . . but anyone running 75-mph in a pickup truck is too dumb for words. Can't change high center of gravity. At 65 it should do well. A pickup should be geared for no more than 55-65 mph anyway. With work to be done this is the high range of travel speed.
The .5T with an AEROLID bed topper could come close to -- even break -- 20-mpg with one of the 25' or smaller AIRSTREAM TT's given a travel speed of under 60-mph.
Certainly, it will be (for a truck) an easy one to live with while solo. I'd say that is the main selling point.
.