Use gray to flush black

The question wasn't about using gray water to flush out the black tank, but using it to flush the toilet. I think I've read here of one or two people who have rigged something up to do that. It's basically just installing a suitable pump and running some water lines to it. Probably having a selector valve to let either the fresh or gray water be used for flushing (and a check valve for the fresh water line to prevent any possible cross-contamination from the gray water) would be a decent idea in case one needs to go potty before anyone has taken a shower or washed the dishes.

Note that the gray water can get pretty stinky over time. For the amount of fresh water that would be saved, I don't think I would bother. RV toilets don't need a lot of water to flush.