While removing the electrical and water reels from the 345 this morning I noticed this female hose fitting attached to the side wall of the compartment. Tracing the hose I found that it is attached to about a 2" round flange on the deep end side of the black water tank.
My only guess is that it is a flush line of some sort. I'm also guessing that the round flange on the side of the black water tank is actually some sort of check valve or at least I hope that's what it is, otherwise you'd have a nice flow of something back into the compartment
So the question is, is this an Airstream installed product or is it an aftermarket item added by some previous owner?