air going somewhere.....

You air gauges do not read any air leakage before the main reservoirs, so leaks at the compressor, wet tank, and dryer do not count. Leaks there would cause the compressor to cycle. If the governor is leaking past the unloaders you have to remove the air inlet hose and listen in it.
I use soap and water mix in a spray bottle and check each and every connection and fitting. Open the dash, and check there also.
The suspension valve has a delay of about 2 seconds but if parked where there is a lot of wind it will drain the tanks.
Try changing the parking position so it is into the wind.
The fan hub, there is a plug at the front and you can reach around and feel it wet your hand and you should feel the cooling effect. Also the seals, are accessible, on most units.
Listening threw a 1/2 inch heater hose works for some leaks.