supplemental braking and out of state

With all the talk about brake requirement laws, braking distance standards, or supplemental braking system standards, I note that there are NO "braking systems" being sold for RV/toad use. In ALL cases, the systems sold are devices that APPLY THE EXISTING VEHICLE BRAKING SYSTEM. It's that factory supplied brake system that must meet the applicable state and federal standards. The external device that applies that braking system has no such requirements. It doesn't matter if the external applying force comes from a cable system, an air or vacuum cylinder, or even a HUMAN LEG/FOOT. In fact, in every RV/toad setup I've ever seen, it's a HUMAN LEG/FOOT (ignoring some handicapped driving setups) that initiates the braking action. Everything else involved is just some mechanical/electrical means of transferring that action to the brake shoes or pads. I don't think anyone can dispute the basic premise that the more wheels that are applying braking force, the easier it will be to stop the vehicle or combination.