Make sure the top of the sewer hookups are not six inches above ground level -- that stuff does not flow uphill well at all! Keep them as close to ground level as you can.
I have even seen RV parks where the sewer opening was slightly lower than ground level and located within a recessed concrete basin, like a sani-dump. That setup, while more expensive, greatly reduces the chances of a sewer spill.
And of course you know that a site contaminated by a bad spill can't be rented out for several days, while everything is cleaned up and dries out . . . . .
Good luck with your park!!
