I'm going to say that it is going to really depend on which one you like best. For what you are towing the 6.4 will do just fine even in the hills. It's just going to rev and shift more. Fuel mileage I doubt will be that much different. The edge will go to the Cummins on that but not a lot. I guess what I'm saying is that in the long run there won't be much difference in the cost of the two trucks as far as ownership. Reliability is hard to say. The 6.4 is still fairly new in the trucks but seems to be doing very well so far. The Cummins is pretty much self explanatory.
I don't think you will go wrong either way you decide just keep in mind that you may decide to grow your RV over time and the heavier you go the more you will like the Cummins in the hills. The exhaust brake is worth it's weight in gold in the hills.
