Well after 6 years and no air leaks to speak of, I have my 1st air leak that I can not find.

Symptoms:
1. Key off
2. Level low in drive position
3. parked on a level spot
4. Air bags holding air, NO LEANS
5. Primary and secondary air gauges lose about 1.5 lbs an hour.
6. They will eventually go down to zero over a period of 3 days or so.
7. They go down in sync. Both primary and secondary seem to lose air at the same rate.
8. Aux Air guage goes down to about 70 Lbs and holds there.

I have sprayed soapy water all over the air tanks, their fittings and drain valves with no luck. I have to believe that since both tanks are going down at the same rate that there must be something in common with both that I need to check.

I am thinking that it could be:
1. Air compressor ?
2. Air governor which I have been unable to find...??
3. Air dryer?

Any others I should look at? And how do I test the above 3 components?
It is a very small but persistent leak and I may have to wait until it gets bigger to find it, but I don't like that idea. What makes me frustrated about this leak, is that until now the primary and secondary air gauges NEVER lost so much as a pound over weeks or months. Those gauges were rock solid at whatever pressure was on the system when I shut down the bus.

Any ideas where to look?