I'm certain that the towing with the RV is the contributing factor in the failure. The amount of bouncing that I see in the mirrors is astonishing and this certainly puts a significant number of cycles at higher loads on the tongue, than a car, that can lead to either a fatigue or catastrophic failure.

Tom's 10.9 bolts are rated at 145,000 psi so much closer to the imperial grade 8 bolt that I used as replacements than the original 8.8 bolts that were installed in my trailer. These are close to an imperial grade 5 bolt.

So from Tom's experience, I conclude if you tow with an RV and have 8.8 bolts replace them!