Some guys claim to get phenomenal mileage. I had a 2011 Duramax LML and saw 14.5 mixed/empty and 10.something towing.
I did a 1200 mile round trip from my place to the Grand Canyona a year ago, empty. I did mostly 80-85 the whole way on I-25 and then 40 going west. Solid 23 mpg on my 2018 Duramax. Going 60- 65 with a 17K total load, I get a tad less than 14 mpg.
