I would not close on the park until you have the following items in hand:
1) A Certificate of Zoning from the city showing how many lots the park is allowed to have and whether the park is 1) legal conforming 2) legal non-conforming (grandfathered) or 3) illegal. This brackets down their argument substantially.
2) A roster of any outstanding code violations from the inspector (typically none). This brackets them down even further.
3) An understanding from the city that they are going to respect your property rights under grandfathering, including bringing in homes any time you want.
Not having these items in hand is as dangerous as not having a Phase I. If you have to go to court, the legal fees could easily exceed $25,000 to $50,000, but even worse, you might lose and the park is doomed.
Don't let a city push you around -- definitely bring in a lawyer and press for your rights. But don't proceed until it has been resolved.