Office for sale of mobile homes, as well as the sale of fixtures and structures customarily incidental to the use, maintenance, servicing and well-being of mobile homes located on the same site. Retail sales, including sales of bottled gas, are prohibited. The area devoted to such sale shall not exceed one acre of the site area, and any area devoted to such use shall be deducted from the site area for computation of the residential density for the mobile home park.