I camp a lot of places without water or with limited water. I usually hike a lot and especially in the summer I need a daily shower. Drinking, cooking, washing dishes and the daily shower adds up to 3 gallons, about 1 gallon is for the shower. Yup that is a Navy shower, wet down, soap up and rinse off. With my wife we run through about 5-6 gallons a day.
You can try to carry extra water, but if you do not conserve you can still empty your tanks within a few days. How much water do you think you could safely carry on your roof? Maybe 20 gallons or so.