Auxiliary air tank

Truck has the 3 factory tanks, and on the rear drivers side by fuel tank is a small 5 gallon air tank. Has an input and out put.
One has a pressure regulator, presumably the output line.
Truck does have air slide 5tth wheel and air pin decoupler, 2 buttons on dash.
Why the pressure regulator on the line?

 

Yes. I should be more specific , what is the tank used for? I dont know that it is for the 5th wheel functions or something else? What would the regulator be protecting?

 

Also another question if anyone knows what is the thread type on the factory tanks? Thought it was 3/8 mpt but it won't thread in

 

Oh, so more of a control valve that opens at a specific pressure rather than a pressure regulator that doesn't allow anything over a specific psi?

 

It's on a 2019 vnl860.
Lines are in a huge bundle of other lines,cables, etc and I loose track of them. would have to cut open a bunch of zip ties and such to separate and follow them but both in and out appear to route towards the front main tanks along the frame rail.
Is it a for sure the regulator is on the output line?