Bodies with conveyor systems not automatically exempt

We sell certain self-unloading trucks to farmers, who use them primarily to haul farm products, such as seed potatoes, sugar beets and fertilizer, both to and on farms. Each of these truck bodies contains a powered conveyor belt and powered rear discharge door, making these vehicles significantly more expensive than a standard truck. A customer advised us that bodies with conveyor systems used to haul fertilizer to and on farms are always exempt from tax under IRC Section 4051. Is the customer correct?