Boatman, you nailed it with "...change my life forever." In a horrible way.I have done research too and have not found any specific cases of insurance denying a claim but I also know how courts work and my fear would be leaving a civil decision in the hands of 9 jurors (usually 9 in a civil cases) where only 5 need to believe the 'victim' to change my life forever. Imagine I'm legal to tow 10,000 and I'm towing 10,500 and I'm in an accident where a car goes through a red light but I hit them and the whole family dies. If a good enough expert witness is brought in and tells the jury that my stopping distance was increased by 10% because of being overweight then the jury finds me 25% at fault in a $10M case then that will change my life forever. I have not seen specific cases of this but don't want to take that chance. If a lady can sue McD's for her spilling her hot coffee on her then...
And yes I was a witness in a civil case where 3 kids were hit by a car on the way to their bus stop. IIRC the driver was 70% liable, the school district was 20% liable and the kids were 10% liable. Might be off by a few % points but that was the ballpark result.
It may cost me a few $1000 more to have the right truck and trailer and to maintain it. I keep records and receipts to prove due diligence. That's so cheap, compared to an accident where I'm found to be negligent. Years of my life gone: investigation, fighting with my insurance company, legal challenges, lawyers, court appearances, conviction and perhaps bankruptcy - and heavy fines or even prison for criminal negligence. And that doesn't begin to address the public humiliation, my possible injuries or the enormous guilt burden of injuring someone else, or worse.
All of that because I was a cheap, irresponsible, intentionally negligent old fool. "Oh, it's okay, an accident is unlikely and my insurance will take care of any problems.", you told yourself. Until your life changed forever.