Interesting story.

This past August I drove a Ford Budget rental truck with a heavily loaded 16' box and while towing a Chevy Malibu on a car dolly from Montana to AZ.

We suspect that truck had a 5.3L gas V-8. With the load and while driven at 60-65 it delivered 10-11 mpg for the 1300 miles. It roared on the hills, struggled in headwinds, was slow to pass the few slower vehicles we encountered... and I really missed my 2500HD Duramax. Mileage wouldn't have been that much better, but the trip would have been more enjoyable.

So... if someone only tows occasionally, a gasser might make sense. They'll get the job done. For those who tow a lot more often - a diesel might make more sense.