WE had a 40 gallon tank in our other TT would last 5 days with 2 adults and 2 kids. You need to take a navy shower. Get wet as fast as possible turn water off. Use as little soap as possible, including shampoo, then hose off. Dish washing is the other. Use as many disposable products as possible,, paper plates cups, plastic knives fork etc. If youhave to use something that needs to be washed, same thing, lightly wet, little soap.
If you change your habits you can extend. Also you can carry spare water.