The last post documented the original toilet in the old SportCam project. That post also described extra gate valves under the kitchen sink. Those valves allowed the sink drain to bypass the holding tank when desired. Instead of storing dirty water onboard, the drain path could be diverted directly outside so dish water could be collected in a bucket. That gray water could then be used to fill the toilet reservoir, instead of flushing with the limited drinking water. That was the concept.
This post documents that creative plumbing in more detail, including some pix.