When I added the inside the spring bags to my old Roadmaster wagon, I used on of these.
https://www.airliftcompany.com/shop/26161/
It has the built in gauge and a paddle switch that connects to source of air and exhaust, so you can fill or bleed from the switch. It is full shut off in the center.
I plumbed the source air to a Shrader valve that I mounted on the same panel. Outlet to the bags.
It is very easy to use as there is never any of the air loss that Marko mentions. You put the air hose onto the valve, and nothing happens until you move the switch to the fill side, allowing air to the bag and showing on the gauge. When you release the switch, the bags are isolated, so when you remove the air hose, no loss of bag air. If you get them too full, you can bleed off some with or without the air hose on.
It makes it very convenient, as you can also just look in and see if the bags have held air and are still where you want them.
We have the double switch, two pressure gauge version in the van so we can set sides individually.