Leak in inaccessible area in '89 Sovereign

[Kevzenn]

Hello. I just bought this airstream and everything seems to be in great working order with exception of a small leak in rear left bath if you below toilet. It s only when I erin water down the sinks i think but I hear a spray and water drips out side where the sewer connection it. How do I set into that with out cutting into the exterior metal?