[NH_Bulldog]

The fitting on top of the black tank is pre-installed and the pipe fits down into it from above. The easiest way to check is to get up on the roof, pull the vent cap off and shine a light down the pipe and into the tank.

Another couple of thoughts; have you checked the AAV under the sink for proper seal and operation? Depending on floorplan configuration, some sinks and showers drain into the black tank either on purpose or by accident. Another thing to check is the toilet seal on the floor.