Coming into this RV game with zero knowledge about US trucks or their engines we got extremely lucky.
We have a 2006 6.6L 6 speed. Bought it at 6 months old with only 16,000 on it. 170,000 miles on it now & it has been about flawless. It has a bad glowplug but I don't care. Don't drive in cold weather. It is a bone stock 2WD dually crewcab longbed dually. It has done some serious pulling. MD to AK 3 times. MD to West Coast 3 times but maintenance wise it is one pampered truck.
Never been to a GM dealership but have a great smalltown garage with a great reputation that gets more or less a blank sheet every spring to go through everything. If in doubt, replace. I treat it like I treat my boats.
Had a weak moment a couple years ago thinking it might be time to replace but concensus here on RV.NET was keep it. As confirmed above it is one of the great model years. Like the looks of the 2006 better than the new ones too.
It is off to Newfoundland in June.