I've got a 2003 Duramax pushing 120,000 miles and so far, it's been the best truck I have owned. 22-24 mpg light on highway at 60 mph, and 14-16 mpg towing my 22. We are moving up to a 25 next summer so I get to see the difference. I agree with B~C on the reduction in mileage with different emission controls. Also Chevrolet/GMC kept raising the HP/Torque rating of the engine in the spec war between manufacturers. Every HP increase had an effect on mileage. I found I didn't need any more power, empty reaching 85 mph was pretty quick (only did that once), and pulling the 22 hit 70 mph once (making the Whittier Tunnel).
Tom
Tom