"I heard that some Keystone FWs did plumb the bathroom sink to drain into the black tank..."
Our Forest River MB Cruiser has this "feature" as well. I figured it out the hard way after the bathroom sink was left running. You can imagine what happened when I went to flush...
I've learned to live with that "feature" now, and actually try to make use of it. If I need to dump, but there's not quite enough water in the black tank, it's pretty clean and easy for me to add water to it. It beats standing there with my foot on the pedal.