[Guy]

Our coach, being much older than 431 has little in common with black water tank design, but we nearly removed a tank that we were sure was leaking when it has a few inches of liquid in it. I always have to remember Richards, "look for the simple first" mantra. Liquid was coming out from under the tank, but the slow ( 1 cup per hour) stopped with the dump gate valve in the open position. After wraping different components with blue paper towels. we found that the 3" dump vavle on the drivers side had a leak around the operating shaft. It would wick down the abs drain pipe, and drip off the bottom of the tank, out of our line of sight. 

I don't know if any of this applies to the design of your newer coach.