So many red flags ignored here it isn't funny. But it is kind of funny. This never should have happened, from the owner, to the initial surveyor, to any real estate agents or title companies involved. Add in any park employees who never said "boo" that a house large enough that no one could ignore it was being built on this location? And this was built by a developer? Really?
The lesson here is thus: Partner with an experience Commercial RE agent who understands zoning, and has a vast number of reliable managed resources to whom to turn to make sure a client in a development situation has the data and knowledge they need ahead of time to stay out of trouble.
Toss this under the heading "Check It Out," even it you think everything is alright. Sort of like someone handing you a sidearm and telling you its unloaded. Firearms Etiquette -- and common sense safety -- says check it yourself anyway.