The ride height valves are operating all of the time while moving. They may be leaking more at that time.
Does the pressure climb and drop while sitting still with the parking brake off ? ( button in )
If it does, that system may have a leak. To look for that, you need to chock the wheels, push in the park brake button and get under looked for leaks.
Once the air dryer purges, it sits empty. Even a leak on the signal ( small ) line won't drop the guages. There is a 3rd tank ( actually the first, called wet tank ) that doesn't have a gauge. Any thing leaking in that system can cause constant purging but not gauge dropping due to check valves.
Check that parking brake circuit.