Permitted Locations of Trucks and Truck Trailers. It shall be unlawful for any person to place, keep or maintain, or permit to be placed, kept or maintained, any truck or truck trailer in any residential zoning district of the City, except for pickup or delivery service, or public agency or utility company vehicle while used on official business, or as part of the operation of a legally existing nonconforming use; provided that in no case shall such truck or truck trailer be parked at any location in the residential zoning district longer than required for such pickup, delivery service, and/or official business, other than within the grounds of a legally existing, nonconforming use located within the residential zoning district and of which operation of the truck or trailer is a part.