If you expand your business or want more capacity, what you developed looks exactly like the "Potato Bed" trucks I used to drive in my youth. Although we used ours for seed wheat, the bed was designed for potato handling.
It is basically identical to what you made, except it was powered via PTO, not a separate engine. Slab sided bed with lower third a v-bottom, and a conveyor belt base. You could control flow by conveyor speed and hatch opening rate.
I bet you could get a pretty used cheap potato bed truck if you wanted something dedicated. Look in ID and WA as this is where I have seen them most often.