6.7 mpg on DD15

Try disconnecting the battery and see if it helps, no joke.
Good luck if it isn't throwing any codes.

 

How much your truck idled?
My truck equipped with smart AC/Heater powered by batteries, and zero idle.

 

DEF usage is directly proportional to mpg. So yes, you use more in the winter than summer. I used 14 gals of DEF in one day when the winds were in excess of 60 mph and I was getting 2.2 mpg pulling super b trains (140,000 lbs).
My DD15 gets 5.11 US mpg pulling these loads in the summer, current 30 day average is 4.08 US mpg. and yes thats about okay for what I do. One of our guys had a T800 with a big Cat, he only got 3.75 in the summer, but he drove like it too.

I think you are going too fast for your gearing. Mine does 1520 RPM at that speed with 3.91's, an 18 spd and 11R24.5 tires. But I don't go that fast. I cruise at 100 kph (62.5 mph) at 1410 rpm.

 

That's about right for your speed and running cruise all the time, GVW above 70k.

 

I have 190000 miles on a 2015
6.9 mpg overall 75 mph truck
you want 7.5-8.0 drive like your trailer says PRIME

 

put low resistance tires on truck and inflate it 105 psi. also try fueling at ta or petro. with love's fuel you will not get good mpg. if you have 6.7 wintertime you will have 7.2-7.5 summertime.

 

I have same numbers. 2012 dd15 462k m. however dd15at engine has slightly better fuel economy.