Although I am one of those who believes that a secondary or auxiliary braking system is important and effective ,I still have a problem finding any legitimate statutes that specifically address the legal requirements for doing so!
There are all kinds of statements published by the various companies selling auxiliary braking units indicating regulations in this jurisdiction or the other but no government issued requirements can be found probably because Towing equipment is not specifically covered under legislation.
If someone can provide such documentation provided by the department of transport I would be interested in reading it!
As stated I use a proportionate braking device which works very well with the air braking system in my Diesel Pusher and my Honda CRV !