Replaced EVERYTHING!!!!!! Except the tanks. BOTH regulators, all the tank hoses, every fitting that connects ANYTHING, AND THE PIECE OF BLACK PIPE THAT RUNS ACROSS THE TRAILER! The piece of black pipe cost as much as any other piece in the entire system!
BUT, I carefully applied the yellow teflon tape AND the proper pipe goop, put everything together. MOVED the 2-stage regulator up a bit so the propane tank DOESN'T hit the regulator when it's in place (YES, the trailer came with the main regulator positioned such that the passenger side propane tank banged it all the time.
After 4 days with BOTH tanks open and switching between them, I no longer smell any propane in either tank compartment OR the compartment between them. Hopefully, it's fixed.
So much for the service people at Noble RV that REFUSED to test for leaks 'cause "It's been modified. It's a bomb waiting to go off."
WHOLE THING cost less than $100, which is a LOT cheaper than making the 50 mile trip BACK to the dealer and giving them a blank check to go prospecting. I SUSPECT it was the 2-stage regulator, but I figured it was simple to just replace EVERYTHING and be done with it.