Drop the belly cover and check the (Black Tank) area! Your BT vent is located thru the (Top side) and out the Roof,it should have exited the roof vent,unless it (Disconnected) from the Tank. The (Tank Rinser) nozzle or Hose could have (Disconnected also. Since it is (Almost Clean) NOW,drop and check locations of possible leak. Did I say NOW,not after it turns (BLACK)!!
Youroo!!