I'm a little late to this, but I can give you my experience with dump trucks. I have a mid-80's GM70 medium duty with a 14' dump bed, 366ci gas V8, and 5spd with 2spd rear. First of all, it has absolutely zero creature comforts. No A/C, minimal heat, and rides like, well, a 1980's dump truck. I've used it to haul 16' logs and gravel up to just over 8 tons (max total weight around 30-31k lbs), and it would average right at 5 mpg. It absolutely hates any sort of hill with a load, but does OK on level ground...you're just not going to get anywhere fast. It does OK empty, but it rarely goes anywhere without a load in the bed...so you really have to use the 2spd rear. I'll admit, it isn't that much fun to drive if hills are involved. It simply doesn't have enough power to maintain respectable speeds, and if you lose momentum on any incline, you're rocking 2nd/3rd gear at 20mph until you reach the top. Don't get me wrong, its been a fantastic addition for my needs. Bed height is perfect for my Kubota L48 backhoe, and it has been dead solid reliable for the last 5-6 yrs I've owned it. But there is no way I would want to work out of it day in/day out with a load.
Comparably, I've pulled similar weights with an '07 Ford F550 6.0L diesel (total weight w/ trailer 32k lbs) and a '14 Dodge 2500 6.7l diesel (30k lbs total weight). The F550 averaged just over 9mpg, and the Dodge would do 12mpg. The Dodge had, by far, the most power, but the F550 felt the most stable (and had the better brakes).
Short of a newer gas truck with a big block, I wouldn't want any older dump without a diesel. Mine would be absolutely perfect with a 12v Cummins shoehorned in and turned up to 275-300hp.