The exact thing happened to our brand new Dutchmen this year on our first trip out. While flushing the black tank using the outside flush connection, water been coming out from under the side of the TT under the black tank flush connection. I found the water commi from inside the TT, under the bathroom sink. There is a back flow valve that is either faulty or connected backwards (I still haven't had a chance to test it again). I would start there. After researching it I found that that is actually a common problem with black tank flushes.
Mine leaked too. The hose fittings were lose. I tightened with a set of pliers, and they haven't leaked since. I imagined they were only hand tight from the factory and the road vibration loosened them enough to leak.