Derby6, with a 12,500 tow cap what would you suggest he use for a weight limit? I agree that they can do more than rated, but you also have to consider thatliability may be be in case of accident. HAte thinking about accidents, but consoder car pulls in front, he doesn't stop in time, he is over cap because 'we all know we can pull more than cap'. What do you think the insurance is going to do, plus the guy he hits lawyers?
As an RV salesman I know the myth out there is that we all tell folks 'sure, you can pull it', but not so. At the dealership I work at (occasionally), we have a safety statement, and if there is not sufficient excess capacity, we won't let you drive it away, and only sell it if we deliver because it will stay, or you are replacing your truck. Customer signs a statement allowing that we explained it was not safe. In case of accident, we at least have this form to cover us. I can't tell you how many times I have had to explain that he should not pull that trailer with that truck.
I hear ALL the stories, oh, my buddy does it all the time, or the truck salesman told me I could haul a house, (he meant dog house I am sure, he just 'forgot ' to say dog....
Another item to consider, just cuz you can pull it doesn't mean you can stop it safely, or that the cooling system, the transmission system, the tire ratings are all up to the task.
All boils down to, careful what you tell someone thy can do with what they have.