Read my lips.... no fifth wheels with a f150/1500 series truck. Unless it is 20 feet or less long. Its not a matter of towing, but rather a matter of loading. A typical fifth wheel will load approx 20% of its loaded weight directly over the trucks rear axle. 1500 type trucks tend to have P metric tires with very low load carrying capacity.
To prove my point, load your truck up like your going to go camping and drive across a set of scales. Subtract your scaled weight from the GVWR as posted on your drivers door post.