Are you considering a gooseneck adapter for the the 5er? If so, and if you have a new trailer, be aware that SOME trailer manufacturers will void your warranty for using a gooseneck adapter. If you are not under warranty, many owners do use such adapters with no apparent problem.
But as mentioned above, you can get the B&W Companion 5th wheel hitch that fits that gooseneck ball opening. And the hitch is relatively easy to remove when not using to give a clean bed.