OK, I replaced both U-brackets on my hitch and I advise other owners to go check theirs if they have a lot of miles.
See larger image at:
http://tour.airstreamlife.com/weblog...des%20rust.jpg
I talked to some more knowledgeable folks about this and they said that if the part failed due to metal fatigue, I'd see a rust line where the metal has been breaking for some time. In the photo above, you can see a clear rust line on both of the broken sides of the U-bracket, as well as sharp shiny steel where it finally broke.
You can also see severe stretching of the holes where the pin ties this U-bracket to the screw jack. This part needed replacement.
See larger image at
http://tour.airstreamlife.com/weblog...%20bracket.jpg
You can see the same metal fatigue rust line in the second photo, which shows the remains of the bottom part of the bracket. Also note distortion in the plate from the last bits of metal bending before they broke.
See larger image at http://tour.airstreamlife.com/weblog...%20bracket.jpg
The last photo (above) shows the other U-bracket. At first glance it appears OK. It's very hard to see in this photo, but the upper corner has a distinct crack in it. It was already in the process of failing! This crack is so subtle that you would not notice it in a casual inspection. You really have to look closely.
I will be replacing both U-brackets annually from now on.