I had a 06 2500 and now have an 08 3500 DRW.
My 06 got about three MPG better, but was over wieght limit. (Never trust a sales person)
The new truck/5r just come in at the limit for wieght. I understand the difference in mileage is caused by the partical filter high pressure differential (plugging) which then causes the computer to inject diesel while the exhaust valves are open. A little raw diesel to burn the particles out of the filter. This happens to me about once per tank of fuel. I'm not thinking the mileage will get much better.
I was told today that all brands of trucks are doing the same thing.
Did I mention don't trust a salesman.