Do you guys have any issues with the 6.7?  

I know that a friend of mine has the exact same truck as me for his work but his is a 5.9 gasser and he said the best mpg hes ever seen was 12mpg.  I get over 20mpg with my 5.9 cummins  

Take it from me I have owned 3 gassers and I towed day to day just like you.

One truck was a 1992 1500 Chevy reg. cab short bed with a 4.3L V6 5-spd with a 3.08 in the rear. I got 18mpg and 8mpg towing only 5k but there was alot of drag due to the fold up ramp, it got toasted due to a speaker catching on fire and it burned to the ground in front of a fire station.

Second truck was a 1990 1500 Chevy 4x4 ext. cab long bed with a 5.7L V8 auto with a 3.48 in the rear. I got 17mpg and 9mpg towing 8k. Still have the truck tranny died at 215,000 miles.It has a built 300hp 355 in it now with only 30k on the motor.

Third truck was a 1999 Dodge 1500 ext. cab short bed with a 5.2L V8 auto with a 3.21 in the rear. I got 17mpg and 7mpg towing 8k inclosed trailer.

Now I have a 2002 Ford F-250 ext. cab long bed with a 7.3L V8 diesel 6-spd with a 3.73 in the rear. I get 20mpg and 15mpg towing 8k inclosed trailer.

All of my gas trucks started to smoke burn oil ect... Except for the 1990 Chevy that has a new engine in it now

But if you tow day in and day out get a diesel powered truck nuff said.