Trucks GVWR rating from the drivers door post minus your actual ready to camp scaled weight will give you the real world payload without exceeding the manufacturers rating. Again in the real world a typical fifth wheel will have a pin weight between 18 and 25% of it's actual loaded ready to travel weight. Since that number is fluid and subject to change on any given day. Most people base pin weight at 20% of a trailers GVWR. You my never actually see the trailers GVWR on a typical trip, but you will always error on the side of caution. Which is a good thing.
OR, you can do like a lot of folks and go buy what you want, load it to the gunnels and go camping blissfully ignorant until something serious happens like a blown tire which could cause all sorts of problems besides the damage to your trailer.
Your choice!
