I agree to a point that some people put up some outrageous mpg claims but I don't see any of that in this thread, they all seem like reasonable reports to me.
When I had my 5.7 2500 with 3.73's, I averaged 12-12.5 mpg's with a city/hwy mix not towing. I can't remember what my towing or straight hwy mpg's were.
Anyway, the 6.4 should be getting a bit better than the 5.7 and it also depends on where you are towing, I am in Colorado and used my truck between 5 and 10,000 feet so mpg's do not get much worse than where I am at.
The link provided is actually pretty indicative of what the members in this thread are reporting.