The 6-inch RV holding tank you received measuring 21 inches wide instead of the advertised 20 inches is not the correct size for your 20-inch wide space. Elkhart RV Parts offers a 30-day return policy for items in unused condition, except for some custom or special order products which may be non-refundable. You can initiate a return request via their online return portal. If the product was sent incorrectly, the company may cover the return shipping costs. Regarding the 20-inch wide by 70-inch long tank you need, the customer service team can assist you in locating the correct tank that fits your specified dimensions. It is recommended to contact Elkhart RV Parts customer service directly to confirm availability and arrange a replacement or exchange. This ensures you receive a tank that meets your size requirements and is compatible with your RV.