Despite cargo capacity, vehicles primarily designed for towing are taxable

We sell at retail certain vehicles that have GVWRs of 32,000 pounds and are often used to tow trailers. However, these vehicles also have some cargo-carrying capacity. A customer advised me that Internal Revenue Service rulings state that if a vehicle has any cargo-carrying capacity, it cannot be treated as a tractor for purposes of IRC Section 4051 tax. Is the customer correct?