Our camper has a sink and water tank, but we rarely use it because there isn’t a grey water tank. Instead, we use a spouted water container meant for a refrigerator. It sets outside the camper on a little table, often with a bar of soap in a plastic box next to it.
This is potable water that we also use for cooking. It’s there to put a fire out, mostly before bed. If there is any left in the AM, I do a little happy dance sine I don’t have to go find water. I fill a percolator and heat water for oatmeal and hot chocolate, then my beloved coffee. Done!
If you are in need of setting up a hand washing station, the Girl Scouts use a large Tide container with a spout. They tie a knee-high nylon filled with a bar if soap to the handle. It works just fine for a group.