School me on 5th wheel hitches please

A ball in the bed of the truck is called a goose neck hitch (Maybe you already know that) not a fifth wheel hitch. There are adapters but they are not necessarily a good idea. Like adding a pipe to the handle of a wrench the goose neck adapter adds leverage to the frame of the FW trailer that it likely was never designed to take.
Personally, when looking at a used FW, if it has a GN adapter I immediately discount it from consideration. I don't need hidden frame cracks.