There are some wrong answers on here. The correct answer is "whatever the septic system is designed for". You can have one septic system designed for the entire park, unless it's really huge. I looked at a park that had 65 homes, all on one septic system. The septic system was only 5 years old, so it was recently designed, permitted and installed.
I took a septic design class in MO years ago, used to have a book on it. There are dozens and dozens of approved system designs.
As far as pumping goes, really Randy has the most correct answer. Residential systems it's generally recommended to have them pumped every 3-5 years, but this is just a generalized recommendation. One little old lady needs pumped less often than a family of 6 with 4 girls flushing sanitary napkins.
Personally, I'd prefer the one system per park. Easier to maintain IMHO.