RV Repair: holding tank leak, rubber grommet, p traps

Question
We recently purchased a '00 Fleetwood TT.  After setting it at our permanent site, we hooked up the water, flushed the lines and everything appeared ok.  Then, the first time we ran water in the tub and drained it, we noticed that the holding tank underneath was leaking badly.  The leak is not detectable by any visible hole, but rather is coming from the fitting or where the tub drains into the holding tank.  In other words, when I look at the tank from the underside, I see water running around the edges of the tank and then dripping onto the ground.  This also happens when we drain water from the sink drain as well.  Any help or advice as to what it would take to verify the leak source and possible repair solution would be appreciated.  Thanks!!

Answer
Leaks are sometimes hard to trace. I like to verify if the leak is above the tank by finding all the p-traps on the coach and removing all the access panels to inspect. If no leaks there I move on. On top of each tank you will find 1 inlet from the drains and 1 inlet to the roof exhaust vent. This is where I have found 90% of grey water leaks. It is a rubber grommet that tends to crack. And they are no fun to replace.