I Have owned three different GM trucks all with the 6L it is a very strong reliable engine in my opinion...
2000 Sierra 2500 Ext cab SLE approx 11MPG
2004 Escalade ESV Approx 16MPG
2007 Silverado 2500 HD Crew LTZ Approx 13MPG
All the same block but different mileage, all numbers are city driving not towing