I first heard about these in an old 'trailer life' mag of the 60s?? I think.
This seems to be the answer for stopping, which these heavier motorhomes seem to be prone to...

http://www.telmausa.com/telma_htm/default.htm

Yeah, they are pretty neat, but way expensive. That is how diesel locomotives and streetcars slow down, too.

It would be fun to work that unit into a regenerative system, whereby the electricity generated slowing down could charge the batteries. Of course, it would require AGM batteries to take full advantage of the high currents available. The bigger your battery bank, the longer you could wait after setting up camp before you had to run the generator. If the batteries got full, then they could be used to power the driveshaft motor/generator unit up steep hills. This would save fuel and result in lower transmission temperatures.

Yeah, we need a hybrid Barth.

Continuing in the vein of strange braking systems, I seem to recall a short popular science blurb where a ford dually pickup truck (or something similar) was outfitted with a large air tank in between the framerails. As the vehicle slowed an compressor attached to the drive shaft pressurized the tank, and then when the brakes were released that pressure initially propelled the vehicle to 30 MPH, or something like that. Supposed to save a bunch of fuel in town.

I wonder what ever happened to that idea...

Edit: Holy cow, here it is. I can't believe I found it. Chalk another one up to Google.

The 350 horsepower hydraulic-assisted, hot-looking Tonka Truck

this is basically for my further information....
Frenelsa Retarders at:

http://www.frenelsa.es/en/producto-aplicaciones/como-funciona.asp