A quick Google of RVing with Large Families turned up this. There are more....
http://ourtravelingtribe.com/https://www.kelloggshow.com/about/Also keep in mind that camping is allowed at many fairgrounds - if a event is not going on at that time. We've used them a lot as we traveled. A lot of them have hookups and dump station along with restrooms. Here's the Mason County Fairgrounds, Michigan web site. There are many more in Michigan... and everywhere.
https://www.masoncountyfairgrounds.com/non-fair/camping/Also wondering what your RV is that sleeps so many! Good luck.