Pulling with a Ram 2500

I have taken it on a couple of trips, the furthest was over 300 miles and avg. 8-10, but that was when I had 1000 miles on it. I was informed that the Ram Hemi get better mileage on longer trips and or the truck is broke in around 8000 miles.. Just getting an idea from owners with a similar set up is all. I have not pulled it with-out the WDH yet, and since I store it several miles from the house I hate to hook up drive 50 miles and drive back to find out but it does make sense to do that first. Now I am not going to cancel my trip just trying to map out all the details. My silverado with the same set up with the 5.3L V-8 only got 4-7 mpgs with a 25 gallon tank so its safe to say I was stopping an awful lot, especially when I went down to Houston. The Ram has a 31 gallon tank, so I am hoping not to stop as many times. Again I hope.