We have five 6 gallon jugs and use them all the time to refill our 30 gallon tank. And we get by on 5 or 6 gallons a day while boondocking, showering every night, washing dishes, toilet, etc. DW is very good about taking efficient showers.
Someone asked why one would shower every day while camping. First, an old question -- is RVing really camping?? My trailer is tiny, but it is luxurious compared to tent camping -- fridge, stove, bathroom, decent bed, a place to sit, etc.
But second, I get pretty dirty every day while we are hiking. It is a big treat to come back at the end of the day, take a shower, change out of our mostly synthetic hiking gear, change into comfortable cotton tee shirts and shorts, and kick back with cold drinks and snacks before dinnertime, enjoying the total silence of an isolated campsite.
But I guess that not everyone enjoys that same feeling -- and that's why they make ice cream in several flavors!
Someone else mentioned gray water as a limiting factor -- we use buckets to transfer gray water into our toilet and thus into the black water tank. There is a lot of unused capacity in the black tank! This technique enables us to go a week or more without dumping the tanks, while adding fresh water to the fresh tank as needed.