I've been RVing since the late 50s. AS a direct answer to your suggestion that we got along just fine with no guidance as to weight....I have witnessed many an accident whose likely cause was overloading. I've seen broken trailer frames, broken hitches and burned out engines and clutches or transmissions and even a twisted axle or two (one of them mine) on these older tow vehicles. I've also seen many of them that stopped much like a freight train, taking double the NORMAL stopping distance but had NOT had that accident (yet).
The DOT stepped in because circumstances begged for some sort of regulation since people often showed little sense in what they towed or carried. It's pretty much the same thing as the seat belt laws. Everyone says "we got along just fine" without them but you don't hear from many of those who did NOT get along just fine without them.