Never heard of that. Your holding tanks have no common connection with your potable water tank sooooo, unless a black tank flush line was improperly plumbed, I cannot imaging how you could accomplish that feat. The closest you get to fresh water and the black tank is the toilet bowl. However, even if the water flush valve was open, you would still have to open the bowl valve. I will remain amazed and mystified.
The recipe for perpetual ignorance is:
Be satisfied with your opinions
and content with your knowledge.