When I was looking at the gasoline engines, the 6.8 V10 was still superior to the 6.2 V8 in towing but is only available in chassis cabs and gets worse mileage. The 6.4 Hemi seemed to be the compromise between the two Ford engines.
If you're on the western side of WA, you will hit mountain passes often. Even the coastal range out to the OR coast is a work out. Without knowing how much you are towing and how often, it's hard to make a recommendation.
I was strongly leaning toward a gasoline engine, but my demands put it too close the maximum capacity of the gasoline engines and I went back to another diesel (22-23K lbs current GCW and will be increasing). If I was towing less weight or less often or on flatter ground, I would have chosen the 6.8 or 6.4 gassers.