I would immediately look up the zoning of your park, and what is allowed by that zoning.
It may be your park is zoned mobile home park (or was when constructed) and that zoning does not allow for RVs.
My first line of defense would be to contact the city, and let them know that RVs create additional income for improvements and upgrades to the park, and it will be difficult to maintain the property to the same level without RVs. I would also try to find out what instigated them to suddenly start pushing back on this.
I have a bunch of empty lots in an area with high RV demand, and I may have a similar run in with the city at some point.