Grey water is what comes out of the sink drain and shower. There is no human waste or harmful chemicals in grey water.
In our van design there are two greywater tanks. One collects water from the shower, the other from the sink. Each is connected to an electronic ball valve. This allows the tank to be emptied at the flip of a switch.
Black water tank
Black water is water that contains human waste, and harmful chemicals. No need for a black water tank, because we use a composting toilet.
Water tanks can be a pain. It is often difficult to find just the right size. I created a video showing the dimensions under the promaster van. This should help!
Electronic ball valve https://amzn.to/3H2HEIk
These are magical. They will allow you to flip a switch and drain your grey tank.