I believe that most states have different rules and tax consequences for businesses that operate RV parks/resorts as opposed to mobile home/"trailer" parks. The distinguishing factor is usually that RV parks cater to a transient customer group as opposed to a "resident" clientele or those living permanently or semi-permanently on a particular site.
Similarly, we've owned sites within an RV resort in several states that have had rules of staying no longer than 180-consecutive days, etc. and the rules are set by the underlying development to comply with local ordinances pertaining to what type of development they are classified as.
I don't doubt that California has the regulation that the owner/management of the park you're citing is referring to. It probably has to do with the distinctions and definitions of "Mobilehome Residency Law and Recreational Vehicle Occupancy Law."
Summary of the Mobilehome Residency Law and Recreational Vehicle Occupancy Law
Complete synopsis of the 2016 California Mobilehome Residency Law
I haven't read through all of it but I have to believe that somewhere within those documents, it will discuss what that "nine month occupancy" is all about and how it applies to their business.
My opinion is that the RV park is free to set its own rules in order to comply with whatever business category they wish to fall under pertaining to California law. I guess it's your choice to patronize the business under their rules or not sadly to say. I feel your frustration.