Bathroom flood - What did I do wrong?

restorodder wrote:
We have a 2008 Newmar Canyon Star. Yesterday was our last day on the road home from Florida, so I wanted to empty the gray and black tanks. First I opened the black tank dump valve, and let it empty. I then opened the gray tank dump valve, and let it drain. Next I turned on the black tank flush valve, both dumps are open. While I was waiting for the black tank flush, I turned the fresh water intake valve to the "fill tank" position. I wanted to make sure we had at least half a tank of fresh water. After a few minutes, my wife came running out, yelling the bath room was flooding. I shut off both water hoses, and went in to find water all over the floor. The top drawer of the vanity was very wet. I could see water running down, what looked like a vent stack,( 2" black plastic pipe"). After I mopped up, everything seem normal. The water appeared to be clean, not gray water. I can't figure out what caused the flood. The fresh water tank was not over filled.

I never leave the gray water valve open when flushing the black tank, not that it should have backed up through there and blowing past your sink trap and you'd think with that being the case, it would come up through the sink drain as well. Anyway my procedure is to empty the black tank, pick up the hose to make sure it's clear. Close the dump valve for just a bit while turning the flush valve for filling. Then open the dump valve and letting it go while continuing some more with the flushing. Close both of those valves and open the gray which should have some water in it and hopefully some soap suds as well. A lot of this depends on whether or not you're staying or leaving and if you're just dumping or flushing the black tank. If just dumping, I like having some soapy water in the gray tank and like I said, open only one valve at a time, beginning with the black tank.