Those rates seem somewhat on the high end to me. We have a park where 4,000 gallons of just water would run you about $20 (direct billed). Another where 4,000 gallons of water/sewer would run you roughly $41. However, we also have a park where 4,000 gallons of water/sewer would run you nearly $107. The rest are between $41 and $107.
Have you considered billing back just the water as your "rent raise"? Our parks where we have sub-metered average roughly 3,000-3,500 gallons of usage per unit. It cuts your cost $45 per unit but doesn't raise their living expenses by that same amount. You could always come in the next year and start billing for sewer if you wanted to split it up between two years.