Things I learned about water in my Sprinter van:
3 ounces of water is enough to dampen a wash cloth to use on your critical areas between showers. When it's cold out, put the water in the microwave for 30 seconds before pouring it on your cloth.
3 ounces of water in a cup is enough to brush your teeth and rinse your brush.
A bowl in the sink will let your dampen your hands for washing then rinse them multiple times if you don't go overboard with the soap. It only takes a drop of a liquid soap to do the job unless you've been greasing your bearings or something like that. For those jobs, wearing disposable gloves removes the need for extra hand washing.
Our daughter went to an environmentally conscious camp and the sign in the bathroom said, "If it's yellow, let it mellow. If it's brown, flush it down."
If you put used TP in an air tight bag, you won't fill up your black tank as quickly.