If the place is hooked to municipal sewer, the park is probably charged for water and sewer combined in one payment. In other words, the more water used, the more goes down the sewer so they get one bill for both.
I don't get free municipal water at home and I would not expect to get it at an RV park as a full timer. Now for a short term stay, I would guess that it is figured in the daily rate. Basically if you use public water and sewer someone has to pay for it. The park is making the ones that use the most, pay the most. Sounds fair to me.