Opinions needed on Tow truck - Diesel or Gas?

[69Chevelle L78Deluxe300]

I have done a lot of towing of my boat and trailer that weigh 10,000#. I live in Phoenix AZ so when I pull it out of the valley I climb up to 5-7000 ft altitude. When I bought the boat I had a 3/4 ton Chev Suburban with a 454 that was rated for 10,000# towing, but that must be in Nebraska, not anywhere with mountains. It would go about 15MPH up the hills and I got about 6MPG . I put on headers with some minor improvement, then I put on a Paxton supercharger. It then had plenty of power but after about 30 seconds pulling up a hill it would overheat and I was back down to 10 MPH . The most I could tow in AZ was about 4000#. I sold the Suburban and bought a new Ford 3/4 ton crew cab pickup with the Power Stroke Diesel, 5sp manual, and 4.10 gears. This is the only Ford I have ever owned, but it is the best tow vehicle I have ever driven! It will pull up the hills at 50MPH with the air conditioning on in 110 degree summer heat!! I even towed the boat to RI and back to AZ and averaged 12.8 MPG for the 6000 mile trip at 63MPH! I put the truck and boat on a truck scale and the rig weighed 17,100#! It averages just under 17 MPG when not towing. After my experiences, I would never get another gas vehicle to tow.
Jim