I have a 1998 12 valve 5-spd, a 2010 6.7 automatic, and have had a 2001 manual and a 2005 manual. The only ones that get (got) reasonable mileage is the '98 and the '01. The '98 can reliably pull down 18-20. The NV4500 did the traditional 5th gear failure but there are good fixes out there. On a good day the '10 6.7 might, might, get about 14 mpg on the highway turning about 1900 rpm. Power out the wazoo but you pay for it. If I was going to do straight highway hauling I'd still go with the Cummins, but for around town and short trip, I'm with Grizzly, a gas rig makes more sense.

(Notice I haven't even mention DEF which is another drawback to the current versions)