That 3000 pound limit for braking usually applies to trailers, and it depends on the state in which you are traveling. some states are less, some states say GVWR of the trailer, some states say unladen weight of the trailer. Many states have a legal definition of "trailer" that clearly excludes towed motor vehicles.
Yes, the laws of physics are usually immutable. One of those laws, that is rarely discussed, is "Reaction Time". That is, the time that passes between when the driver realizes that he must stop NOW, and the time when his foot actually touches the brake pedal. It is shocking how far a vehicle will travel in that period of time at 65 MPH!
Another law of physics is "Stopping Distance". That is, the distance the vehicle will travel AFTER the driver slams on the brakes. At 65 MPH, that too, is a shocking distance.
In the majority of sudden emergency scenarios often presented on this forum, when the Reaction Time is added to the Stopping (or Braking) distance, it becomes obvious that an empty motorhome will not stop in time, much less one loaded near GVWR.
Google it. The results can be quite educational.
All that having been said, is auxiliary braking for a towed motor vehicle a "good idea"? Yes, of course it is. Just don't expect too much of it.
Now, the towed vehicle...
PT Cruiser with manual transmission (automatic can be dolly towed)
Subaru with manual transmission (automatic must be trailered)
Jeep Wrangler (not exactly inexpensive, but they can be found for under $10K)
Geo Tracker 4X4
Suzuki Samurai 4X4
VW original bug, Baja bug, Manx, Thing, or Karmann Ghia (again, a little pricey, but can be found for under $10K)
Some automatic transmission vehicles can be towed four down with the addition of a lube pump. Check with REMCO to see if your choice is on their list.
CM1, USN (RET)
2017 Jayco TT
Daily Driver: '14 Subaru Outback
1998 Dodge QC LWB, Cummins, 5 speed, 4X2
2 Kawasaki Brute Force 750 ATVs.
Pride Raptor 3 wheeled off-road capable mobility scooter
"When seconds count, help is only minutes away!"