I installed the "missing" nut (standard 1/2-13) on our TT in it's first season of use. I only discovered it because the propane regulator was installed backwards and you could not see the indicator that says it's switched over from the empty tank (just another stupid thing they do). So I turned it around and in the process found the vertical threaded rod flopping around.
Why wouldn't they install one to begin with? Well, to quote the dealer we bought our TT from "it's just a travel trailer" when discussing various issues we were having. There's many things on TTs that make you ask yourself "what the fig?". Today I get to replace the crappy WFCO converter that died a few days ago with a new PD one. I could probably write a book on all the ridiculous things I've found on the 3 TTs we've owned...
BTW, when you add the missing nut, it becomes a jamb nut and you only need to adequately tighten it up against the existing one.