I am one that beleives that the term "half ton towable" is a very misleading term and a marketing gimmick.
As mentioned above, 20 percent pin weight on a lighter fifth wheel (say gvwr of even 9000 lbs) will be around 1800 lbs. Add 250 for hitch and hitch mounting hardware, another 300 for passengers and 300 for other items in the truck and you are loading approximately 2600 lbs onto the truck. This will clearly exceed most 1/2 ton payload ratings. Most 1/2 tons also are equipped with P rated tires. In my opinion, they are too soft for that kind of load.
I would research thoroughly before buying a fifth wheel for a 1/2 ton.