Madcow, A lot of the receivers that are installed are not bought off the shelf.They are fabricated to fit and carry as much weight as you can put to them. One of the hitches I have uses a 4 1/2"X 4 1/2 x.500" cross beam. There is no rating on it.You will fold the back of the frame rails over before the hitch fails.The truck this hitch is on also has doubled frame rails that go from the rear bumper to about 18" past the front of the rear spring hangers. I have bent the 1/2 or 3/4x 3 flat stock on the 2" square hitch part many times before.I now use a cast iron hitch that I have never bent yet. I have trailered around 42000lb on a 2 5/16 ball and hitch assembly for several years.
The hitch assembly that is used to move Mobile homes is not much different than the one I use or the ones that are fabricated for real trucks that are capable of handling the weight.