Replacement fifth wheels not taxable

Our dealership sells taxable tractors, truck bodies and chassis to a large trucking company. On occasion, one of our salesmen will take an order for a tractor (outfitted with a fifth wheel) and for one or more fifth wheels that will be used by the customer for other vehicles it already owns. We’ve been told we must charge the 12% FET on parts and accessories that we sell along with a taxable tractor or truck. Is this true?