My Wife and I are new to the camping adventure so bare with me on this one. We have used our 2004 keystone outback 28BHS about three times this summer and every time when I dump the black and grey tan...
Take a peek under your trailer and see if you can see the tanks. On mine I thought they drained out the side, but found out that they actually drain out of the center.
My black tank drains fast, my gray tank sometimes seems to drain really slowly. I'm suspicious that there is something like the piece of plastic they drilled out of the tank still sitting in there and sometimes plugging it up.