In the Coach Assist section, there is a 1996 Monterey Air System diagram (part copied below).
Note that there is a "quick disconnect" port on the air tank. So if you don't have an air pressure gauge on the dash, you can buy an air pressure gauge (like 0-150 psi) and a matching quick disconnect coupler and plug it into the tank to verify the air pressure. If you are building up air pressure to 120 psi then move to the next troubleshooting step. If your not getting full air pressure then check for a bad air compressor governor, problems with air compressor, external leak, etc.
Good luck