You definitely have more than one leak. To simplify things you need to break it into groups or systems. Ill try to clarify. From the compressor check valve to the fill valves is one system meaning its all at the same pressure (this includes the traps, gauge, pressure switch etc.) From each fill valves to the bag is also one system at the same pressure. Does that make sense? Now leak test one group at a time and ignore the rest. I would suggest starting at the tanks and get that squared away bc it could be causing your rear to be dropping. This is why.
I see you have check valves between the tanks and I am guessing the fill valves. That is usually used for 2 reasons. Some valves require higher pressure on the inlet side for the valve to stay closed. When the tank leaks the the bags back fill the tank till they're both empty. In cars where cylinders or small bags are used, the pressure in the bag would build up as the suspension was compressed and the same thing would happen. The rear check valve is either leaking externally and bleeding out the air or just isn't sealing. The reason I say that is bc the front should be exactly the same and its not leaking overnight. You could have leaks on both rear bag systems, but since you say you went over everything ill assume you didn't miss anything. Are you using push to connect fittings? You could swap the lines from the check valves to the front and rear systems and see if that moves the leak. That would isolate the check valve or rear system leak.
If you want to check the rear bags. Spray the entire bag down as well as the end caps, crimps and fittings. I have had bags leak at the crimps as well as the mounting bolts.
The water traps are going to be harder to check for leaks but I try to put check valves between the tank and traps. When plumbing them like you have any leaks at the trap will drain the system. I only use the SMC all metal and had customers forget to fully tighten the drain. You have a lot going on there. DONT ASSUME anything isnt leaking just bc it is new. Check the gauge, pressure switch and check valves methodically and you will find the leak.
Sorry for the long post.