I drive an '04 Dodge 2500 4x4 with a Hemi. It's got a 6 inch lift on 37" tires. The best I've ever seen was 14 on a road trip to Biloxi with my girl driving most of the time. Most of the time I get 9. something, but I drive the hell out of it. Pulling a trailer I've seen as low as 8 going from Cham Tucker to Gainesville. Runs about $90 to fill it up.
I've heard that diesels are not that much better, only with calm long hwy trips will you see high teens...
Towing capacity is great with the Hemi. I can pull 13k no problem.... and I can stop too, most of the time...
Durability, you got me there... I don't expect mine to live much past 60k miles...

Good luck.