The type of joists makes no difference. The bottom line is after you buy a mobile home, the value decreases. It's the same as buying a car. The older a mobile home is, the less it's worth. The same with buying a car. The minute you drive it off the lot, the value decreases and the older a car is, the less it's worth.
Mobile homes is not real estate. It is considered personal property. A mobile home has to be permanently attached to the land underneath to be legally considered real estate. And the mobile home owner must have a title or deed for the land underneath.