The law and courts are not as black and white as some people make it out to be. There are items like "due diligence" and "intent" that are taken into account in this type of civil case. If it went that far the manufacturer and the insurance company itself will probably be involved because the defendant will start stating they were "told" the truck could handle it and direct the blame to Ram and that the insurance company should cover the cost of an accident. Besides, the manufacturer and insurance company would be where the big money is. Sure, you can sue anybody for $20 million but even if you win where do you think the money is coming from? The average blue collar Joe Schmoe simply doesn't have that much money to give, and the courts simply are not going to literally take everything from a person and quick them out on the streets...doesn't work that way.
But back to the comment....so if a civil case comes up you are talking years before it would go to court. So you are saying somebody is going to immediately impound the truck and trailer combination and lock it up for years so they can go back and weigh it? Then go weigh the passengers and any gear that happened to be in it. Keep in mind you are talking about a totaled out truck and trailer combo that was scattered across the road.
In the end, especially if using the OP as an example, you are going to have a hard time winning a civil case against somebody for pulling a 3,900 lb. trailer in a truck that is rated at 2x to 3x that towing capacity simply based on being over payload rating by a few hundred pounds.