So around here diesel is about $3.50 and gas is about 2.80, $0.70 difference.
I pull an 11,000# 5er with less than 300hp, I very seldom get slowed down by a hill, with 3.55's. then with the Pacbrake I very seldom need to touch the brakes on a downhill.
Pulling the hills to the Coast and keeping at between 55 and 60 get 12 mpg, I will not tow with gas again.
My old 460 got maybe 7 mpg, so I am saving money on fuel. That is not the point, the point is the comfort of towing with less than 300 hp, and knowing I will not be held up by a hill.