The first problem is to determine what that alarm represents. Even though in the dash, it may have triggers other than the air system. Other things that could be wired to the alarm include jacks-down, tv antenna up, etc. Any of them could be real or a false alarm.
If it is air-related, it could still be a false alarm. Ask Freightliner Custom Chassis Customer Service (1-800-FTL-HELP) where low air is sensed (I think it is at the gauges) and how to verify whether it is working or not.
Former owner of 2004 American Tradition and several other RVs
Home is in the Ocala Nat'l Forest near Ocala, FL
Summers in Black Mountain, NC