I'm with brholt on this one. I think there are a lot of well meaning people that when you ask them about fuel mileage, they tell you the best they ever got, or the complete believe the trip computer, or they are really bad at math. And there are also a significant amount of people who just downright exaggerate.
I think the best way to get a real comparison would be to check fuelly for the specific vehicles you are considering. At least that way the numbers should be hand calculated, and they are over hundreds of thousands of miles with many users instead of just a one off message on a forum.
That said, I don't think there is a significant difference in any of the current HD offering in regard to MPG. The RAM 6.4 will be similar to the Ford 6.2 which is similar to the GM 6.0. Find the interior that works the best for you, and don't worry about the small difference in MPG.