"Supplemental brake are highly recommended in every case to reduce the emergency stopping distance."
Seems to me there is a big difference between "highly recommended" and "required". The statement I was referring to said "Most states REQUIRE..."; this is not true when referring to towed motor vehicles.
Plus, the fact remains, if you can meet the FMVSS and/or your state braking performance requirements, YOU ARE LEGAL, period!
The most common standards are to be able to stop from 20 MPH on a level, hard, clean surface within 40 or 45 feet. Montana is 40 feet, California, IIRC, is 45 feet, FMVSS, IIRC, is 40 feet. That is, for ANY "combination of vehicles".