Black water tank not draining completely

Sounds like an obstruction, sometimes when they cut the vent and drain holes they leave the plug in the tank (foolishly) and it lodges when you drain....Drain it again look down the bowl with a flashlight, send a hose in from the discharge end, just start looking.....also check your vent on the roof, make sure it is clear, flush the tank if you have a connection. One last thing, ignore the sensor, they never work correctly ....