You most likely have an engine brake instead of an exhaust brake on your ISL, but I cannot be certain. Still, you should call Cummins, with your engine serial number, to confirm if you have the engine brake and ask them if it can be programmed as you describe. Call Spartan and ask the same questions.
Larry, Eileen, and Finley
2004 Alpine 36FDDS
Third motor home, first Alpine, no need for another.