The best way to stretch water is through conservation. We use a non-toxic liquid soap and use a dry(er) wash method for dishes. We also don't cook things like pasta that wastes water, unless you like drinking pasta water as friends of ours do. For washing, we pre-wipe everything with a paper towel moving from the least dirty to the worst. If water is needed we use just a splash then transfer it through all of the dishes as we work our way through. We then soap our sponge and start thoroughly washing from smallest to largest item, again if water is needed we transfer it along. Usually we just wipe the dishes dry with a clean paper towel and keep re-using it until it becomes saturated, then relegating it to being a pre-wiper or table/counter/stove wiper. If we have a stubborn pot or pan, we always add a little water and heat it on the stove to loosen up the debris and scrape it with a spatula etc, we only use stainless which makes that easy. We usually only carry 10 gallons, sometimes 15, and can easily go about 5 days on 10. We drink a lot of water, and coffee.
If you are near water, non-potable is fine for doing dishes and washing yourself. If you are concerned you could always boil it. We have even washed dishes with ocean water, no issues.