Picked up my 2500 6.7L Cummins Mega Cab 4x4 in St Louis on 17 July and drove to Oxnard, CA via Ft. Gibson, OK, Plano, TX, Yuma, AZ, and San Diego, CA...averaged 19.6 MPG! I used the cruise control as much as possible. A/C was on the whole way. Exhaust break was on most of the time (Didn't have to touch my breaks at all driving down the mountains towards Phoenix in Northeast Arizona!) There where five of us in the cab with about 400 lbs of luggage in the bed.
Now, mostly driving in town, it is down to 15.7 MPG. She is mostly stock. No mods to motor, trans, or emissions. I ordered a RetraxPro cover from fastNugly which I installed while still in St Louis. Maybe that helped my highway MPG. Thanks fastNugly!
...oh, and no regen until I reached 2919 miles.
No issues at all during the trip.