Yeah totally agree, that 10k rating thing is just about registration and driver's license requirements, nothing to do wtih safety and the truck's actual capability. It's purposely "derated."
I don't know where you live but I can tell you when it comes to weight ratings, Kansas Highway Patrol only looks at the total of the tow vehicle GVW and the trailer GVW. Given the GVW's of most 5th wheels, you would have to load things pretty heavy to go past the total of those 2 ratings even with a 2500. Guys towing campers don't get weighed but IF they did, the officer will be looking at your tire weight ratings, right off the sidewall of the tire. They will write a guy a ticket for "unlawful/unsafe tire", meaning a tire is overloaded. The tire ratings get you more in the neighborhood of that 12k number that has been mentioned, depending on your tires. That's why many times when you ask for loading advice, guys will tell you to just worry about staying within your tire weight ratings. This is fairly universal nationwide although there are a handful of states that I would never speak for. Call your own state police and see what they say.