I'm not sure that there is an average RV? Different RVs have a different numbers of people in them, and each of them probably use a different amount of water than any other RVer uses. And each of use do things differently at different times. When we park where we are dry camped, with no external water supply, it is much different than when we are connected to city water. And then if dry camped, for how long? If for just one night, most conserve very little. But if we plan to stay for a week, or even longer, then we go to great lengths to conserve.
For anyone to give you any usefull information, you need to give us more information about what you are looking for?