Friend of mine hauls apples with a Dodge tandem axle and a swapped 378 V6 with a 13 speed Roadranger.
Last season he said mileage was 2-3 MPG average for loaded to the plant and empty to the orchard.
I'll have to ask the weight again, but it was as much or a little more than a tandem axle truck can legally haul, it's got a pretty long flat bed and they stack the bins.
A 378 will haul most anything with the right gears and plenty of fuel....