Regarding Supplemental brakes--I can tell you from personal experience that you want them. I had a breakaway this summer where my front bumper separated from the towbar on one side. This was a very scary moment. I was able to get the vehicle stopped using the one remaining connection, but this was a struggle to do while going down hill on a 2 lane highway. The jeep was in the oncoming lane, and required me to move over to shoulder a lot more than I wanted to. Had it broke away at that point, it would have killed someone in oncoming traffic.
I don't care about the laws, rules, or any other reasoning--Supplemental brakes are just common sense and will save a life.
If my vehicle killed someone because I was too cheap to do it right, I don't think I could live with myself
I am DEFINITELY no expert on this, but I understood supplemental braking requirements had to be met for the specific state you are driving in. Your drivers license requirements fall under the reciprocity agreements but I don't believe that applies to requirements for supplemental braking systems.
Requirements for supplemental braking is required for some states, but each of those states have different weight limits for which they are required. Example, Michigan doesn’t require supplemental braking until the toad is over 10000 lbs.