I am just a weekend camper. I may go out for 4 days. With the girlfriend, one of her grand kid, and myslef I have never had a problem with filling up the gray or black tank. They do use up my small fresh water tank so I just refill it with a 5 gallon Coleman Jug that you can purchase most anywhere.
The first thing you should do it to use water conservaton. The less water you use the less you have to haul. Take Navy Showers, use paper plates, save some of your dirty dishes and wash them when you get back home. A gallon saved here and a gallon saved there adds up.
