Finding an exact replacement grey holding tank with the specified dimensions of 72" length, 29" width, and 12" height can be challenging, especially if the original tank is obsolete. We recommend reviewing available holding tanks on our website filtered by these dimensions, but please note that no tanks exactly matching those measurements may be currently in stock. If there is absolutely no wiggle room in dimensions, options become very limited. Sometimes leftover obsolete tanks may be available in our warehouse, but this is not guaranteed. If possible, consider if any minor dimension flexibility exists to expand your options. Additionally, checking for a part number stamped or imprinted on the existing tank can help with cross-referencing compatible replacements. For fresh water tanks with a maximum height of 5", we have a limited selection that might work if height is the only strict measurement. We encourage contacting us with any part numbers or additional details for personalized assistance in finding the best possible or alternative.