[Richard]

Serious Kudo's to you. You obviously have studied the system, done the research, and carefully explained the behavior before you asked the question!

Bravo!!!!

Next, an infomercial. I prefer using the forum over phone calls. If we use the forum, you get the benefit of many contributors. Everyone learns. The information is here for the next guy or gal if they take the time to use the search engine. No one is going to flame you on this forum for any question, we maintain our decorum. If I help via phone call then only you get the benefit. That being said, stuck on the side of the road is another situation altogether.

I do not know the specifics of your coach or how a previous owner may have modified it. However following Newell logic and DOT mandated design, the answer to your primary tanks staying up, and the supply tank going to zero is easy. In the air system fed by the engine air compressor, the first device is a pressure protection valve that will divert ALL air flow to the primary and secondary brake tanks until the pressure reaches around 60 psi. At 60 psi, the PPV opens and all three tanks (sometimes more) fill at the same rate. The primary and secondary brake tanks are equipped with check valves to prevent air from flowing backwards from the tanks to replenish a leaking supply side.

Without knowing the setpoints of the thomas compressors, it is difficult to say if they should run when the engine is running or not. If the engine has topped off the brake tanks at 120, the Thomas compressor should not run. However, IF you have a massive leak in the supply side, it could leak down fast enough to trigger the Thomas based upon it's setpoint, before the engine compressor kicked back in at 90.

I believe, but this is not confirmed by me, that with the dual Thomas setup, that those compressors do supply air to both the supply ( slide seals, potty, doors, leveling) and the brake tanks. On the older coaches with 12V compressors there was a check valve to prevent the tiny 12V compressor from trying to inflate all those systems.

With your supply side going to zero overnight you have some work to do. Bubble solution and ultrasonic listening devices are your friend. Tons of posts on here about chasing air leaks.