The frames are springy and the jacks are not made of heavy steel either. The Fifth Wheels and Trailers want to move back and forth and side to side as well. The leaf springs want to move side to side too. The stabilizers you put between the tires and lock tight and the additional stabilizer available for fifth wheel pin hitch help to stabilize them too. Some newer fifth wheels and maybe TTs have HWH or other jacks systems and they are more stable.
Former neighbor has a 45 ft fifth wheel, I think and it feels solid.
If the units had air brakes it would lock the wheels and stop a lot of that but i don't think any of them come that way. Can't lock the magnetic brakes as they would get too hot and drain the batteries.
Panhard rods perhaps might help too?
Separate jacks and pads under the frame would probably do it.