We made it to memphis this winter (1200 miles) with only one fill up on Allison's '06 Jetta TDI. Fully loaded and going 75-80 the whole way. One stint at 100+ for a while where it was pretty wide open. We got 43MPG calculated wit GPS and all.
It's quick too (chipped).
I like the power and the economy.
No idea what a gas vehicle would have done in the same conditions.. maybe half the mileage?
On the way back we were tracking similar mileage in the Jetta and I was following in a subaru (2000 Legacy) struggling to get like 20mpg. We were pushing pretty hard but still, the jetta was getting decen mileage.
Not sure if diesel tow rigs double mileage over gas counterparts but I bet they get close. That's not the only attraction though... diesels seem easier to drive and work less hard to do the same stuff... kinda translates to less stressful driving IMHOP.