We just traded our Hemi for an Avalanche after 11000 miles and 7 months...10mpg city, 15 hwy.
I had a friend tell me today that he gets 20 mpg hwy and about 16 city with his.
I still have my CTD HO Auto that I bought a few weeks after the Hemi just for towing. 20 mpg city and about 23 hwy...I got about 12 mpg towing 11000 lbs. The thing with the dually is that 60-65 is optimum speed. If I went 75mph towing my mileage would drop down to about 9.5 mpg. The first tank I got 11 mpg and at about 10000 miles the mileage really jumps.
I would highly recommend the new CTD because in the long run you will come out way ahead....