A park model may or may not be considered a vehicle - check the laws in your state. If it is classified as a vehicle, it will have a title and you must get it transferred into your name, using whatever procedure the state requires (may or may not include a notary). If you don't, you do not "own" the park model at all! If not classed as a vehicle, there may be some other category with its own rules for legal transfer.
Some people refer to a "destination RV" as a park model, but they are technically different. Destination RVs are sized for highway travel without special permits, i.e. they are no more than 8'6" wide when set for highway travel. Park models are wider than that and require wide load permits to be moved, so not really a highway vehicle. Some states allow a semi-permanent but still portable building to be designated as either a vehicle or real property
If the land it sits on is private (you are buying the land as well as the park model), you should get a deed for the land. If a rental lot, that doesn't apply but make sure the landlord knows he has a new tenant, has your contact info, etc.