Robert, your 1991 has the same air system as mine. There are numerous locations which are subject to leakage. In general, there are two major air systems in your coach, auxiliary air and the brake air. So the first question I would ask is, of the two gauges (1 needle on 1, 2 needles on the other) how are they behaving. Do they all go down together, does one brake needle and the auxiliary needles drop together? That will help locate a starting place. A number of us use an ultrasonic leak detector
https://www.amazon.com/Inficon-711-202-G...k+detector to help locate areas of leaks quickly. The solenoids for the air suspension system can be leaking, the fittings for the 120 volt air compressor could be leaking, the holding tanks could be leaking.
So to help you out, you might want to start a new thread and detail exactly what items are/are not holding air and what is affected (leveling, rear air, front air, slide cover, generator extending ram, air seat, etc.
