If you have serious water conservation issues this strikes me as a great idea. Hell half the world flushes their toilets with grey water. And in the case of an Rv it's super easy to separate the grey and fresh water systems, and furthermore there's no reason to have the fresh water system linked at all. On the off chance you're flushing before there's any water in the grey water tank just wash your hands first.
And if you're really squeamish about grey water, put a little bleach down the sink every now and again.
The best water saving feature I've ever done on an Rv was foot switch for my kitchen sink. That way the water only runs when I need it to when washing dishes. Probably doubles my boon docking water time.
