our 04.5 duramax at best did 17 maybe 18 on the highway. Or 2015 denali, 600lbs heavier, does 18-19 consistently, best was 24.5 running about 62 empty when DW was driving the denali and I was driving a uhaul. we won't talk about what the uhaul truck got.!!
drive conservatively on the highway and I think you'll end up over 20.
In town and towing, didn't see much difference compared to the 04.
twice going from portland to san diego on I-5 we managed over 20 hand calculated. newer trucks, considering the huge frontal area, high Cd, and heavy weight do darn good IMHO. In terms of "ton miles/gallon" they are better than most econoboxes.