Keith, My setup is almost identical to what you are considering. I have a 2012 GMC 2500 6.0 with 4:10 gears and I tow a 35' FW. My GCW is 18,200 lb. We took a 2700 mile trip this fall from NY to Tennessee and N Carolina and averaged right at 8 mpg towing.
The only time I really would have liked to have more truck was when we crossed the mountains between Chattanooga and Asheville. I was down to 30 mph on the steepest grades. Also could have used more engine braking on the long downhill runs. (Having never owned a diesel I'm not sure how much better they are.)
Otherwise I have no trouble keeping up with traffic on the interstates.
Dave
