I've been researching this as well for the past week or so.
It's a Parker Pneumatics air dryer for your accessory compressor which has a primary job of performing the auto-level system once the engine is turned off.
I too thought it strange that the solenoid stays hot all the time and is a loose fit on the drain glass.
I traced the power and it runs off of a 40amp circuit tied to the house batteries and is hot all of the time, even with the house battery switch being off. In turn, it has a 15amp circuit to the compressor which I believe is a sense line and power feed so when the compressor is on, it will turn off the solenoid to build pressure.
On a thread from 2014, it discusses testing the air dryer with the engine running and see if you have any leakage as the compressor feeds in to the main air system. Apparently it should have a check valve to the main system to prevent losing main air.
I found the 15amp fused link blown on my compressor so I've been trying to research how it all works.
That dryer is long discontinued and cannot find any wiring info etc., from Parker. HWH packaged it with the auto-leveling system but I have not received a call back from them.
So, I can't be of any help really except to explain what I think is the correct function and how it operates.
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