we have gone 7 days with DW and I, two grandchildren, Daughter and SIL. In 7 days we used just over 60 gallons of water. That included quick showers every other day along with dishes and toilet. For dishes we wash once/day using paper plates and cups as much as possible.
We ALWAYS leave a campsite or home with a FULL water tank. That way when we get to our destination we don't need to hook up when we get there, if there are water problems we don't have to worry. The extra weight of the water makes no measureable difference in towing or in mileage. 400lbs added to 16,000 is in the noise!