I saw a note on eTrailer that mentioned one other thing: high hitching. When you hitch up the bottom of the king pin shout be 1/2 to 1 inch below the skid plate. If you hitch up with the king pin bottom higher that that it's called high hitching. I know that can also cause hitch damage.
They also mention worn king pin but that should not be a problem in your case with a new unit.