Kurt is correct. Any gooseneck hitch or rv style fifth wheel hitch on a medium duty or class 8 truck is going to be fabricated. At best you will find one with a big steel plate that you still have to fab mounts for.
Check these ebay listings for new hitches:
Curt 65500 Flat Plate Gooseneck Hitch 30K GTW Camper | eBay
Curt 61100 Fixed Ball Gooseneck Hitch 30K GTW Camper | eBay
If you are fabbing one up yourself, however strong you think it should be, make it heavier than that and you can't go wrong with too heavy a hitch, but too light will kill you.
Good luck.