Sugestions for Entry Level Cheap Dinghy

Look for a Chevy HHR. A little heavier than you may be looking for but they can be towed 4 down and are reliable as all get out. My '06 has 140k on it and is not showing any signs of needing replacement.

Honda Accord or Civic.

Which brand base plate you'll need will be somewhat dependent on what tow bar you have, although there are inter-brand adapters available in some cases. Your state's laws may not require a supplemental braking system, but the laws of physics do, and they apply everywhere. Your motorhome will take longer to stop when towing an unbraked toad. It may not be much longer, but a braking system could make the difference between a hit or a miss...

Honda CRV before 2014

Can you drive a manual transmission? Older Scion xB with a manual. Less than 3K pounds and huge on the inside. We loved our Scion with our gasser but know that we have our DP, we tow our much heavier Ford Explorer.

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.

Chevrolet Tracker.

A good used Jeep Patriot with manual transmission can be had for a reasonable amount. Other than that, a used Chevy Tracker/Suzuki Sidekick is light and easy to set up for four-down towing.

Another idea would be to find a used tow dolly and pull whatever smaller front wheel drive vehicle you may already have. It could be cheaper than setting up a vehicle with a tow plate, buying the tow bar (if that isn't one on your rig already) and the lights, etc. to make it legal in most states. Several tow dollies have their own lights and surge brakes - the easiest and quickest way to get on the road. Another benefit of a tow dolly is that you can back out (slowly) if needed without having someone in the towed vehicle to stabilize the steering wheel.

I know folks with DP''s that tow and do not have aux brakes. Air brakes IF adjusted right will stop the weights you are at with and without a towd vehicle. Bif thing is to have safety chains installed and utilized and then apply some sever stops to whateverr you have.. try the rv 1st then with the attached vehicle.

Proper adjustment is imperative for airbrakes to be effective. Know how they operate and really understand, they will NOT stop on a dime as many 4wheelers think. You will not stop at many yellow lights if less than 1.5-2 bus lengths away in most instances even without the towd vehicle. So knowing the systems and how they work on the RV will be most important. You will probably be having at least 15Tons of vehicle to stop and they do not in short distances.

Find a older Saturn in decent shape most were good toads.