Is the OP saying every drain goes to the black tank?
...There are some models from different mfgt's where the bathroom sink is dumped to the black tank just to save on plumbing...but not all sinks.
I would suggest a test of each individual drain-line with the black tank valve open at a dump station w/ transparent connection to double check....'cuz the sensors just are not 100% accurate. This will save time with the service request.
To plumb all sink drains to the black would take major modifications to the plumbing lines - so that would be an assembly error that would not easily happen.
(Edit) I would ask questions of others here: http://www.irv2.com/forums/f121/2014...-167193-3.html