Can my rv park deny me my own wifi? ?

To answer your question it would be helpful to make sure we are all talking about the same thing. As Bill-M stated, Wi-Fi does not equal Internet.  Additionally Wi-Fi and "cellular date" are not the same thing.

https://www.cnet.com/how-to/home-networking-explai...

See tutorial above.

Is the RV parking really saying you must use their Internet service?  

If you don't have Internet service delivered to your RV, what good is your own Wi-Fi going to be?

Probably. They may even have good reasons for doing so. Too many people with their own Wi-fi routers will degrade everyone's performance.

Something you need to understand about WIFI.  It is NOT a direct internet connection.

All WIFI does is connect your Computer to a ROUTER and that Router is then connected to the Internet by cable or Satellite. The Cable can be your telephone land line, which is called ADSL;  or it can be you TV COAX Cable or even a Fiber Optic Cable.

What your RV Park has is a WIFI Set up that reaches all parts of the RV Park connecting you to their Router.   Now if you have a Telephone Line or a TV Cable coming into to your RV then you can get your own internet connection. With your own connection you will have a MODEM/ROUTER that will have WIFI.  Another connection method is via the Mobil Telephone Network. The latest Technology for Mobil Phone Internet is 5G.  You can use your CELL Phone and use it as a WIFI Device for your PC.

Do you have contract.  Read it and see if you sign that states that you must use their wi-fi. 

If not how do they know that you are on a private network.  How can they stop you for using your own property?