That Steadyfast system looks pretty nice. It uses the very basic concept of triangulation. Roof trusses in houses, lateral bracing for decks and wall framing use pretty much the same idea.
I know from building decks in the past that when you're near done and the thing wobbles it's worrisome but when you put that long diagonal board at an angle across the underside it instantly stops any movement.
As others mentioned, as long as there are rubber tires touching the ground you'll never get rid of 100% of the movement but that system looks to be a pretty good setup. If I spent more time in my 5th wheel I'd get one for sure. As is, I'm just a weekend warrior and getting bounced out of bed when one my kids coughs is just part of the experience