Well some of this sounds fishy, first the black tank flush system is a closed system with no connections to any other part of the trailer. The line goes from the UDC up the wall to a back flush preventer that is located above any other part of the water system and then to the tank. As far as I can see you must have hooked to the fresh water inlet. If you had to turn a lever you must have turned a by-pass valve, It will be interesting to hear what really happened.