Kevin,
We average 10.5 mpg with our stock '07 chevy srw k3500 duramax diesel at 62 mph optimum. Gross weight 24500#. In hills or headwinds, mileage can be as little as 8, in flat land with a tailwind up to 13. even if the fuel and other costs were higher for the diesel, i'd not give up for the torque. It eats hills and winds that would have a gasser gasping . Of note, We have a need for the truck towing heavy all year 'round, not just for our snowbird time away.
Consider your miles heavy towing against light. It takes a lot of heavy miles to justify the cost of a diesel.
Max
