Korbe, I will admit that the measuring seems a bit odd. But I am trying to evaluate exactly how much water we use, per day, when camping. The real issue isn't the supply of fresh water - it is the capacity of our gray tank, which is only 25 gallons. That has become the limiting factor on our ability to boondock.
So if the two of us can shower on (let's say) three gallons a day -- one for me and two for DW (who still has a very nice head of hair, unlike me) -- and we use a gallon for dishwashing -- then we should be good for six nights without having to find a dump station.
