Sometimes one of the grey water drains may be plumbed into the black water tank. That is the way it is on my coach. The shower and kitchen sink are plumbed into the grey water tank, and the toilet and bathroom sink are plumbed into the black water tank.
It all has to do with accessibility I think, and since the bathroom sink gets used the least, then it does not add a lot to the black water tank capacity.
There should be a vent for each tank. There is on mine, but I don't have any idea about yours. Of course, the easy way to check is to look and see how many vents you have on the roof.
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