So I got an update from the person with the 2017 Duramax.
They now have over 3300 miles on the truck. Lifetime fuel mileage is 17.6 MPG, and that includes idle time using the remote start.
This past weekend on a 400 mile road trip they averaged 18.9 MPG at or over 75MPH. The previous weekend they drove close to 600 miles and got the same average, but drove over 100 miles on two lane roads at 60MPH and averaged nearly 23 MPG for that stretch of roadway.
Overall it seems like they really like the truck so far. Unfortunately they haven't done any towing with it yet. I am on a business trip to the south next week, so I will be searching Craigslist for a rust free southern car so that we can take a road trip and do some towing with it!