We use and Andersen Ultimate rail mount. The first 4,300 miles on our 13,175# (scaled) fifth wheel were with the stock Lippert 1621 pin box without any problems at all (other than the rough ride, of course). At 4,300 miles, we replace the Lippert pin box with a Demco Glide Ride. The Demco pin box is obviously built much heavier and the plate the king pin is mounted to is at least 3 time thicker than on the Lippert. With the under-bed frame mount brackets for the mounting rails on the truck, there hasn't been a hint of flexing or damage to the truck. It seems the OP had an unfortunate cascade of circumstances.
Rob