I am cringing as I am writing this for fear of opening a can of worms, but state laws on braking related to towing is pretty specific to trailers, not cars.
State laws will vary slightly . Not 100% sure but I think most of the towing/braking laws use the word 'vehicle' not so much 'trailer'. Trailers are vehicles and vehicles with motors/engines are considered motor vehicles.
Therefore, it might be worded as such. If it states "a vehicle over 3000# must have an individual braking system while being towed behind an RV" ………...then that would include a car or trailer.