This is common. Cities think they can restrict the land usage of a non-conforming MHP by not allowing those that leave to be replaced. The nature of a MHP is that homes come and go, so by virtue the city has overstepped its authority. You need to get a muni attorney to speak with the city attorney how they handle this situation and get it documented on paper as part of that process that the city will not restrict the existing land usage.
If you have a city that wants to fight it on principle even though wrong, it could cost tens of thousands for you to prevail (and recoup most of it likely years later). And you need to come to agreement in the contract who pays for attorneys fees to resolve it during the feasibility period.
Don't close unless this gets resolved. Why shouldn't you close? What happens if a tornado hits (or insert any other natural disaster) and destroys half the homes. Now you have a 10 pad MHP that the city won't let you infill the vacant lots. What's the park worth now?
Best of luck, let us know how it turns out.