What you found is a real problem, and you found it the right way: walking the property and asking questions until something felt off.
The legal term for what the previous owner made is a neighborly agreement or informal license. Because it was never written down and never recorded on title (meaning never made part of the legal property record at the county), it almost certainly does not bind you as the new owner. You would not inherit the obligation. But I am not a real estate attorney, and before you act on that, you need one to confirm it for your specific state, because property law varies and an attorney is the right person to answer it.
Here is the part worth sitting with before you get there. The neighbor believes the promise is real, even if it has no legal force. If you rent site 14 on day one, you may have a hostile neighbor with a story to tell future guests, local review platforms, or the county permit office. That is an operational risk, not a legal one, and it does not show up on a title search.
On the underwriting side, you have two choices. You model site 14 as permanently dark and decide whether the deal still works at that revenue. Or you model a path to activating it, which includes whatever goodwill or compensation it might take to resolve things with the neighbor, and you price that into your offer.
The strategy guide for RV parks flags operational intensity and location-specific complexity as the factors that separate skilled operators from people who bought a number on a spreadsheet. Site 14 is a small version of exactly that.
One thing I would want to know before going further: did the manager seem to think site 14 was just a quirk, or did she say it like there were other arrangements like it on the property?