Black tank leak?

[Riccochet]

Been using our TT for the past 3 weeks. Appears the black tank leaks slightly only when completely full. I'll see dripping coming from where the pipes exit the belly pan, where they foamed it. Not a lot, like a drip every 15-30 seconds. I tapped around on the belly pan to make sure it wasn't full of nastiness. Once I drain the tank the dripping stops. I can fill the tank to 3/4 full and it's not leaking then. Only when it gets pretty full does it start leaking. I can also smell it under the one sink in the bathroom. Smell also goes away once the tank is drained.

Without tearing apart the underside of the TT any idea what could be leaking? In the meantime I'll keep the tank emptied more often until I can get it to the dealer.