Welcome to Dr. Björn's Auto!
Okay, strictly speaking this is not part of water supply hence it deals with waste water. However it is part of the water circle in your van and we need to think of the whole cycle in whatever we do, don't we?
There is a small waste water tank which fits between left side wheel well and propane tank in a WestfaliaT3 Vanagon. If you do not have one installed then we recommend the mounting kit as well.
There is also a large waste water tank which can be installed on the left side if there is no propane tank installed (measurements in mm).