Fifth Wheel to Gooseneck and need for off-set adapter

Bad idea.....
5th wheel overhang frame and pin support are not designed for stresses caused by the leverage of a gooseneck adapter.

Just compare the structural design, size of frame and supports on a gooseneck trailer vs 5th wheel.

I know they sell them and several folks use them.....but the frame/pin box are just not up to it.

Get a 5th wheel hitch....slider if necessary.