I would say 15 to 16 MPG with the Cummins 5.9 is doable with the load you are pulling.
My Pacific is pushing 11' tall and has a lot of wind drag but the Cummins doesn't know it's there.
The cummins [ 5.9 ] gets the best MPG out of the big 3.
The 07 5.9 has 325 HP and 600ft + foot lbs of tourqe , If your going to tow a bunch I would go with the 6 speed manual tranny.
With any Diesel ya dont want to break 2K on the tach. 2K on my truck [ 2500HD 2WD longbed , the HD has the 11.5" rearend and 3.73 gears ] at 2K I'm running 65 MPH , 2150 rpms is 70 mph, 1900 rpms is about 62 mph.
With my boat in tow I can go from 65 to 80 in a heatbeat , it will set ya back in the seat.
Empty right now I figure 18 to 20 MPG with 245 17E tires, Wayne has the 265's and he turns 1900 rpms at 70 mph [ I'm going 265's when thes wear out and that should give me 2 to 3 mpg more all the way around.
It pulls my boat up the ramp in 1st gear [ 6.21 to 1 ] at an Idle and does not lug at all, it's VERY impressive.
All I can say is " If ya don't want a CUMMINS , don't test drive one "
Here she is at Bluffman2's Casa
