I know the whole insurance thing has been kicked around before and I'm probably restating this. Also I am not nor do I pretend to be a lawyer, I'm just an IT guy so take this with a grain of salt.
If you are doing something illegal, like speeding, running a redlight, failure to put on your turn signal, all of these acts committed fully knowing the consequences in the eyes of the law, and have an accident that is ruled your fault, your insurance is going to pay the claims. Good luck with your rates after that, but they aren't going to say, oh he commited a traffic infraction so we don't have to cover it. Likewise if you have 15 passengers in your 1985 Cherry Red Yugo on the way to the local circus and get into an accident, your insurance is going to pay.
Now if I call my insurance company ahead of time and tell tham I'm hauling the Space Shuttle from Florida to Alaska in my brand new Tundra, they'll probably drop you like a bad habit.
If you have an accident hauling your 5th Wheel and are either over or under weight and there is some catastrophic loss you can bet the lawyers will be lining up, the weight will just give them something else to point out as neglegent behavior. Even under weight there will always be something to find to blame some sort of negligence.