There are several components, in the air suspension system that can leak.
Does the whole coach drop , as if your were dumping the air prior to leveling? Or just one corner?
Do the air gauges read zero when you turn the key on , or is one gauge at 65 PSI, and one in the 100 PSI range?
DO NOT GO under the coach looking for the leak without proper safety stands in place.
Freightliner or Spartan chassis?