I wouldn't do it. The grain trucks around here that I have seen the bottoms aren't made to have rocks dumped into them which with gravel you would have that. Thats a lot of weight being dropped into the bed.
Also call around for hauling rates. We bought a dump truck, single axle due to it was cheaper than having a local guy we were using do it. Then I found a guy that could haul 22 tons per trip with atri-axle dump truck vs our 6 tons. He also got volume discounts at the local quarry. So he was paying 15-20% less than we were. In the end he could bring in 1 load what would take us 3-4 trips and it was cheaper.
Also call around for hauling rates. We bought a dump truck, single axle due to it was cheaper than having a local guy we were using do it. Then I found a guy that could haul 22 tons per trip with atri-axle dump truck vs our 6 tons. He also got volume discounts at the local quarry. So he was paying 15-20% less than we were. In the end he could bring in 1 load what would take us 3-4 trips and it was cheaper.