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Servicing the Big Boy is probably needed, but checking the voltage on each side of the Big Boy will tell you if the contacts inside need cleaning. As stated above, if it is the same on each side, or very close, then the Big Boy is operating correctly. Need to check that first, then you can proceed after you determine what it is doing. If operating correctly, then ignition switch would be next, followed by the relay on the starter Circuit Board in the rear run bay. Another thought, do you normally use the boost mode when starting the coach? Can you hear the Big Boy engage when you hold the boost button?