Spent a frustrating hour trying to replicate the problem -- no luck. I could not develop a tight enough seal for my compressor nozzle.
I am guessing that if I had just tightened the bbq connector while it was still hooked up to the trailer's propane system, I would have discovered the leak was at the joint between the black iron stub and the brass connector. But instead of tightening an already-troublesome and utterly-useless (to me) bbq fitting, I chose to get rid of it and cap off the line.
But even though we don't know the precise origin of this particular leak, at least we've collected some tips and tricks that might save other folks from making the time-consuming errors I made during this search process.
