This video shows you how to make a waste water tank out of a 25 litre container and fitting it along with all necessary pipework under a VW Crafter, also useful for Mercedes Sprinter vans.
Product link list
water container. https://amzn.to/36qpI92
flexi pipe. https://amzn.to/3eLKpAb
22mm ball valve. https://amzn.to/2IkTHao
stainless steel strapping. https://amzn.to/36pn5nQ
32mm waste tank connectors. https://amzn.to/35hwGhn
22mm tank connector. https://amzn.to/32vUNXR