I've got a 1500 Silverado Z71 LTZ with the 3.0 Duramax. I bought it at the beginning of 2020 and have put 35,000 miles on it so far. Mileage has been great - I've averaged right around 27 for the whole time. If I'm driving steady highway miles at moderate speeds, I'll see around 30. At 80 mph or so on the interstate, it is around 25-27. Towing anything has a bigger effect on mileage than with the bigger diesels. I like the engine and the 10-speed tranny combo for my driving. Even with its relatively low hp numbers, the torque is good and its plenty fast.
The truck was problem free until about 33,000 miles. Then the anti-lock brakes and stability control system went out. The dealership replaced the computer control module for that system and I still had problems, so they ended up replacing the brake master cylinder. There was also a problem with the DEF system at the same time that was causing the truck to feel jerky at speed, at it was affecting the fueling. The dealership had to flash the computer control for the DEF system (this is all based on what they told me - its beyond my shade-tree mechanical ability). All was covered under warranty, but doesn't inspire confidence at this point.
Hopefully that will be the end of the issues. 2,000 miles later all seems fine.
