Here's an interesting site for CA brake requirements:
I think what has happened is that the RV manfs have become more safety conscious, for a number of reasons, and have gradually upgraded the trailers.
, for example, used 1 7/8" inch couplers on the 13' and 16', then put 2" on the 16' and now uses 2" on both. Axles on the 13' were 1,200#, then 1,600# and now are 2,200#. I believe Casita
13' brakes were an option and now have brakes as standard. Small Coleman pop-ups now come with brakes standard.
As someone (Brian?) pointed out a while back, regardless of what the tow vehicle and trailer are, six braked tires
on the pavement are more useful than only four...