There are three types of retarding devices used on diesel power motorhomes; exhaust brakes, Jacobs (Jake) engine brakes, and transmission retarders (there are also electric drive line retarders but I don't know of any motorhome manufacturer installing them).
Exhaust brakes, typically used on coaches with diesel engines less than 10 liters, close off the exhaust pipe when activated to apply back pressure to the engine which helps slow the vehicle. These are typically the least effective of the three types but are the less expensive and can be installed on virtually any diesel engine.
Jacobs (Jake) brakes close down the exhaust valves in the engine and literally turn the engine into a giant air compressor. A lot of restraining horsepower is generated by a Jake brake, enough that in 4 stroke diesel engines, typically a switch is provided to reduce the engine braking from working on all 6 cylinders to working on only 3 cylinders to keep you from slowing down too quickly going downhill.
Click Here for drawings of how a Jake works.
A transmission retarder works not on the engine but on the transmission. It reroutes the flow of transmission fluid to a retarder vane provide braking within the transmission itself. These are extremely effective and can generate enormous amounts of retarding horsepower BUT they are VERY expensive and can overheat the transmission quickly under heavy usage so an inexperienced driver can cause damage not possible with either a Jake brake or an exhaust brake as well as potentially lose retarding just as it is needed the most. Transmission retarders are great as long as they are used with a careful eye on the transmission temperature gauge and retarding reduced before tranny temps get out of hand. Foretravel has used Allison transmission retarders for years. Only a few Newells have been built with them and Jimmy and Debbie own one (notice that it was a show coach which frequently incorporate new and interesting features to gauge customer demand).
To make things more confusing Jacobs now markets an exhaust brake but it is not an engine brake.