If you want the exact replacement part, you should download the digital parts book for your trailer to get the part number. Digital copies of Owners Manuals and Parts Books are found at
https://www.airstream.com/owners/manuals/.
On the other hand, just get a length of threaded rod from a hardware store, most likely a much cheaper alternative.
They come in 3' lengths and you may have to cut it down (screw on a nut or two before cutting and use them to clean the threads after cutting and dressing the end with a file). I can't recall the thread size, but I believe it is either 7/16" or 1/2". Bring your wingnut when you go shopping to find the correct thread.
If the fact that the full length threading is an annoyance, purchase a length of thin clear plastic hose with the appropriate inside diameter to cover the threads to match the Airstream original.
I replaced my rod this way when I moved from 30 to 20 lb tanks and the threaded portion did not drop down far enough to move the regulator in-line with the tank outlets.
Greg