I was meaning to post a picture of this for years. As I was blowing out my water lines today I said why not just take a picture.
I took the standard screw on blowout plug and on the nipple extension I cut some threads to match the coupler fitting that matches my air compressors air hoses.
I'm able to attach the plug and air hose to the camper's city water fill location. Adjust the air compressor regulator to around 30 psi and open and close valves on the camper to blow out water.
So there you have it, after all these years I present this. Winter is coming