With an average speed in the mid-30s on my V6, I can typically average about 20-21 MPG. (Mine is 2WD).
I would have hoped for slightly better MPGs from the diesel. However, first tank, be interesting to see after maybe 2-3K miles.
Does "winter-blend" impact diesels like it does gasoline engines? Could we expect better performance in warmer days? Is he using remote start or anything else to drive fuel mileage down? - Granted, that should show up in his average speed. The time spent learning the truck, checking out the acceleration & just getting used to truck will impact mileage. Another reason to await further results down the road.
Finally, and this is not the topic of this thread, how well does the truck perform overall, in regards to the shifting complaints of many? (This is a whole nuther thread I hope the OP addresses, and we need to wait till transmission has learned the driver.