Off-highway use may not mean nontaxable

We manufacture specialized bodies used in the construction industry and install them on chassis that weigh over 33,000 pounds GVW. The use of our trucks on the highway raises roadworthiness issues and may require additional insurance, a specific driving license and other special considerations. Accordingly, some of our customers do not operate these trucks over the highway, choosing instead to haul the trucks to the job site. These customers argue the trucks should not be taxable under IRC § 4051 because they do not use them on the highway. Are our customers correct?