I hope you meant 1.6K and 2.1K. 16,000 and 21,000 rpm is a bit too high!
9.5 mpg isn't bad considering the amount of weight you are pulling. I don't know what the 'sweet spot' is for the Cummins engine, but the Duramax is around 1800 to 1900. It would make sense for the truck to be in it's 'sweet spot' around 65 mph. Does the trans have a tow mode?