Yet another example of how trying to pacify someone in regards to a rule doesn't work.   Over the years I have found it is just as easy to use the two rules that always work.  #1 - My park, my rules.  #2 - Because I said so.  Does it really matter if it is a city rule, a state rule, a federal rule or a park rule?  If they want you to provide a rabies certificate you can either provide one or move on.  But some people just want to object, so they seek justification.  The park provided it by saying it was a city ordinance and that didn't suffice.  There is nothing that is going to make them happy, because even if there was a city law, they would say it doesn't apply to them because it is a cat or it never leaves the RV or they are only transiting the area.  People who are upset with a rule are going to be upset.  That is really the first and last chapter of the book.