One poster referred to stab braking in conjunction with exhaust brake to keep speed under control,
I feel that term (stab) gives the wrong impression of what to do.
Google "snub braking technique" and read and understand several of the references before you head off on your trip. Obviously the main thrust of the articles is towards heavy vehicle drivers and most of those vehicles have engine braking, but the techniques are generally the same for our lighter vehicles fitted with exhaust brakes and automatic transmissions.
One characteristic that does catch some out is they come barrelling over the top of a hill at 60mph, switch the engine/exhaust brake on, lift their foot and can't understand why their rig starts accelerating downhill. The transmission reads 2 or 4 so what is happening. At that point you need to act by applying the brakes firmly until the speed drops enough to allow the transmission to actually drop into a lower gear. Then you assess the situation using snub braking and if the rig still shows a tendency to accelerate alarmingly, then you repeat the slowdown exercise until the transmission drops to the next lower gear and reassess - and so on.
Don't forget, the transmission has only one aim - to protect itself and the engine. You and yours don't count, so if you fail to keep the rig under control by being in the right gear and employing periodic snub braking, then the rig will accelerate to the point where it will shift up towards 6th gear to stop damage to the engine. At that point, you need to hope you have kept the brakes operational so you can do some serious braking to get back to the point where the engine braking plus snub braking can handle the grade.
BTW the reason gas engines aren't normally fitted with exhaust brakes is because they already have a very similar device on the air intake.
Tony Lee - International Grey Nomad. Picasa Album
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RVs. USA - Airstream Cutter; in Australia - MC8 40' DIY Coach conversion & OKA 4x4 MH; in Germany - Hobby Class C; in S America - F350 with 2500 10.6 Bigfoot camper