Any of you ever use a 5th Wheel Dolly like this?

as I (continue to) ponder building a mover, the bracing thing keeps coming back to me. I visualize any motorized mover walking out from under the king pin, so I keep thinking braces from the upright to the frame on each side to triangulate, and then that puts all the push/pull on the frame, rather than the upright support. But, the videos above show trailers being moved just fine without that, so maybe I'm overthinking things again.
I learned farm equipment building and repair from my grandfather, who ALWAYS applied the old german engineering logic of "hellferstout" :)