Elkhart RV Parts does not currently stock RV holding tanks with the exact dimensions of 65" x 21" x 3" that include a fastening lip. However, we offer a variety of gray and black water tanks that may meet similar needs, such as 50" x 21" x 4" or 55" x 21" x 5" tanks. To assist in finding the best fit, we recommend using our Tank Finder tool on our website, which helps locate tanks by dimensions and features. For tanks with custom dimensions and fastening lips, consulting a local RV service provider for fabrication may be necessary. You can order available tanks directly through our website on desktop or mobile devices.