I'm not saying you should blindly disregard factory rated payloads and towing capacity, but my comments. If you are driving down the freeway, lose control causing an accident, and kill somebody.....you are still liable for the accident and death regardless of whether the truck or trailer is overloaded. The "only" additional trouble you could get into, which would mainly being your insurance company refusing to pay, would be if they proved gross negligence on your behalf. I'm not a lawyer but believe proving gross negligence is a lot more than just disregarding factory recommendations. Think about it....you run a stop sign and cause a wreck, which means you broke the law by running the stop sign. Your insurance company doesn't refuse the claim and drop you for that.
Again, not saying to blinding disregard factory ratings and you shouldn't hook up a 18,000 lb. camper to a 1500 truck and go running 80 mph down the freeway. But will say that I question all these people making comments of legal issues. As stated in a reply above there are very few actual examples (assuming what you read on the internet is always true) post of people getting in legal trouble. Not saying it doesn't happen, but just doesn't seem to be that common. I've challenged many people on forums to provide an example of somebody getting in legal trouble and the most I have ever gotten was "I knew somebody who caused an accident and got in trouble because they were overloaded", but when pressed for details they never replied. I think the reason for this is because it's really hard to prove the gross negligence part if this happens, especially if it's not completely blatant (i.e. 18k trailer on a 1500...).