Here’s a link to the manual for your Airstream. The winterizing process is found in the maintenance section.
https://www.airstream.com/wp-content...manual-147.pdf
I don’t know the details of your specific trailer, but the white petcock that you found underneath your trailer is likely the fresh water tank drain. The low point drain valves may be inside the trailer and they may be underneath it. In my case, there are two valves with red handles underneath the trailer, and two additional low point drain valves that are accessed from the inside, behind a panel. Each model is different.