Yep the u bolts going through the bed are for safety chains on gooseneck trailers(I use them all the time). You may check to make sure the draw down bolt ( the gold colored bolt on top of the square tubing that goes down) is torqued to 40 ft lb. And that the u bolts holding the square tubing are torqued to 80 ft lb and tighten each side evenly
You might check that the side bolts are tight too and the bolts on the pin box. Might also check where the pin box meets the frame and make sure there are no cracks.
Hope you can figure it out!