Gray water tank

Ok, the next issue. I seem to be having a leak from the gray water. Don't know yet if its the tank or a fitting. The path it has taken to expose it self is out the rear right where the pipe pops out of the rear of the camper and enters the shut off valve. A few questions, typically how big are these and whats the best way to access it to track the leak down? It leaks early on only after the water has been running on low from the kitchen sink for say maybe 2 minutes so that makes me think its toward the bottom of the tank or a fitting/pipe. From what I can tell from the inside of the camper it looks like the tank is in the center of the camper. See attached picture. Is the best way to access this from under the camper? There is a synthetic burlap type material and it looks like under that a thin sheet of foam insulate type board.

Thanks for your help.
Ed