I can't for the life of me think of any overflows that might be leaking. I would drop the belly panels in the tnak area and look for possible cracks in a gray tank that might show up when the tank is nearly full. I suppose it could leak around a vent pipe which shold be located at the top of the tank.
I had a leak this past weekend when my tank flusher (Tornado) hose blew off the connector at the tank. Clean water
!!! I had to remove a couple of panels so that I could put the hose back on and make sure I got the clamp tight this time.
Anyway, I hope you find the source of the leak.