Except where such parking or storage is directly related to a permitted or lawful nonresidential nonconforming use on that premises, no commercial vehicles having a capacity of more than two tons or having three or more axles, or tractors for hauling trailers, shall be stored, parked or garaged in a Residence District. The parking, storage or garaging of vans, trailers or semitrailers, or similar vehicles designed to be propelled by a separate means of locomotion, or vehicles designed to be used for moving said vans, trailers or semitrailers, as well as all self-propelled machinery designed for commercial use, shall be prohibited in Residence Districts. Customized vans used principally as pleasure or recreational vehicles are exempt from the provisions of this section.