Currently, Elkhart RV Parts does not stock a holding tank with the exact dimensions of 12" x 8" x 84". However, we offer a similar 100-gallon fresh water tank measuring 81" x 26" x 11", made from durable polyethylene and suitable for RV use. We are working towards providing custom-sized tanks in the future but do not offer them at this time. Please contact us to discuss your specific needs or explore available options that may fit your requirements.