You'd be MUCH better off purchasing a B&W Companion 5th wheel hitch which mounts in the B&W Turnoverball gooseneck receiver you already have which will allow you to tow the 5th wheel the way it was designed - with a 5th wheel hitch. Gooseneck adapters impart severe torques into the pinbox area of 5th wheel frames which they are not designed to handle. All that gusseting around the crown of a gooseneck trailer that's there to resist those torques is NOT present in a 5th wheel trailer frame.
Rusty
boy those slide in companions are not cheap.......