And just to kind of re-state what I'm learning is this...
I'll need both a hitch and a mounting system.
A fold flat ball is a mounting system, that some kinds of fiver hitches can mount to. Or I'd need to replace the trailer pin box with a goose box. In that case a hitch is obtained in the goose box and the mounting system is the hitch too (gooseneck).
But another "clean bed" system is the Pullrite Super5th (for my long bed) which is the mount, and it includes the ISR Super rails with removable pins for quick truck camper conversion use.
In such a case I'd buy a new Super5th series mount and rails system, and either a new or used fifth wheel hitch with the standard ISR rail attachment capability.