"Most"!
OH, how I have come to detest that word!
MANY states have braking performance laws, where a combination of vehicles must be able to stop within a certain distance (often 40 or 45 feet) from a certain speed (often 20 MPH) on a clean, dry, hard, level surface. In fact, the states which have such laws take the text almost verbatim from the FMVSS (Federal Motor Vehicle Safety Standards)!
In many states TRAILER laws do NOT apply to towed motor vehicles.
In a panic stop at 60 MPH your motorhome will travel almost 200 feet before your foot hits the brake pedal.
YES, a toad braking system is a good idea. Is it required by law? Probably not, in many states, IF you can meet the Braking Performance Specifications!