I have read a LOT on diesels on Ford's forums. Seems like the 6.7 is a solid choice and now with 4 years of history this is reliable info. Earlier 6.0 and 6.4 suffered from expensive repairs due mostly to the emissions regulations parts.
I chose to go with gas because it's cheaper but also because diesels don't do so well on short trips and I use the truck year round to get the kids to school 5 minutes away from home. For an RV/long trips truck only the 6.7 would be my choice as long as the price difference covers the gas I am saving.