Well I don't know about the rest of it, but $2200 sounds cheap. Or normal.
You will have to work with the county. Get a copy of the relevant ordinances that were in place when the park was built, as well as any updates and amendments which you may be required to adhere to now that the usage has changed from "campground" to MHP.
You may have to get a lawyer to help if you cannot come to terms with the city. Which ordinances apply and which do not? The ones in place when the "park" was developed. You say it was developed as a campground so this seems to be somewhat of a grey area.