Much more weight when towing a STUCK Uhaul truck than pulling a trailer and boat. Plus think about the difference in weight. A 4,000 pound boat and trailer that moves freely or a 12,000 pound Uhaul truck with god knows what in it that is stuck, probably adding another 5000 pounds of weight to it depending on how stuck.
Please don't attach to ball like that. Attach to hitch. Plus the ball is meant to disperse weight from the top into the hitch, not from the side like that. You turn that 3 inch ball into a bullet once that little bolt shears.