30 gallons lasts my wife and I about 7 days when we camp.
1. We shower every other day. We use a small plastic bucket to catch the water that runs out when we set the faucet valves to get the optimum water temperature. This 1/2 gallon or so is used in the wash bucket to lather up and wash our hair....our shower has on/off valve in handle. We then use about 30 seconds of water usage to rinse the soapy mixture off of our selves. Probably use 2.5 gallons max to take a shower.
2. We do not wash dishes. Paper plates all the way that we burn in the campfire each day.
3. We use a 5 gallon GOTT container for drinking water.
4. We use wet wipes to wash hands, our faces, etc..
5. If we do run the sink, we have a plastic container that is used to catch the liquid that we then use to fill the toilet bowl.
6. Grey water is used to put out the camp fire and rinse the ATVs after a long ride.
We carry four 7 gallon water containers available from Wal-Fart. We are able to screw in a garden hose quick disconnect with an inline valve onto the container in lieu of the spigot and then attach a water hose to the tank and gravity fill the fresh water tank for extended days.
Once we quit washing dishes, we realized we can get away with using very little water.
We use a citrus drop in gray water deodorizer and our gray water smells pretty good. However, we often camp by water sources (streams) and can grab a couple of gallons to use for the toilet bowl if we really want to stretch things out more.