My Waste Water Tank is Leaking

[racor]

We are currently living in our coach away from home. This week we left our coach for a three day trip back home. We arrived back at our coach today Saturday, and we noticed a "smell" on the outside of the coach. The inside was normal. I looked around the coach and noticed water dripping from the tank holding compartment hole plug.

I opened the bay door and discovered the "smell" was "black" water in the tank holding compartment?

I looked through both sides but could not see the source of the leak?

Does anyone have any ideas on where to begin to look? And how to locate where the leak may be?

The reason I say "black" water is because presently I have the main dump valve closed, and the pneumatic valve open. I believe this enables all "grey" water to exit directly to the sewer line, and keeps only the "black" water in the tank. I have already emptied the tank just in case I was getting a false reading on my display.

How do I find the leak if indeed it is a leak?