It is true that the bathroom sink drains into the black water tank. I wish there was a way to determine whether the "sensors" are accurate. For example, if it goes from three dots to four does that mean the tank if full? If not, how much is left? Because of the limited capacity, I know for a fact I would not buy this model again. It appears 26 gallons isn't enough capacity if you are dry camping. Imagine what it would be like if you had two or three kids with you instead of only the two of us.