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This sounds like the air system is working correctly, there will be a two wire low air level sensor that will be located off the air parking brake or located on the air tanks, sometimes these sensors can fail or get weak and cause the low air alarm to sound. I would try to find a Cummins Coach Care in your area they should be able to troubleshoot the issue, they deal with the RV application. Most other chassis shops should also be able to located the sensor for the alarm..
To be sure with the coach air built up to 120 psi, step on the foot brake and release the dash brake, the air gage should drop off and then maintain pressure, if the pressure continues to drop and not build you may have an air leak...
IF you have any other questions or concerns please feel free to contact me..
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