If you're talking about a gooseneck adapter that bolts on the trailer's pin box and sits on a gooseneck ball in the bed, DON'T DO IT! Fifth wheel frames are not made to take the torsional stresses that the "lever" of a gooseneck adapter creates. If you're talking about one of the several hitches (B&W Companion, Andersen Ultimate, PullRite UltraLite, etc.) that mount to the truck bed using the gooseneck ball but are not true gooseneck hitches, they are all excellent choices.
You don't mention what year and model your fifth wheel is... just be aware of cab clearance issues pulling certain fifth wheels with a short bed truck without a sliding hitch.
Rob