Yeah, I was worried something would break while trying to unthread the flange. That was before I saw the crack to the underside of the flange. When I saw that I counted my lucky stars it didn't break off.
Over the course of the last year I have read probably every thread on the Monacoers.org forum, irv2.com forum and rvforum.net and everyone just keeps repeating the exact same answers; it's your vent on the roof or the water in the bowl is evaporating. Some tout the need to replace the AAC valves under the sinks, which I had already ruled out, but it's just people repeating everything they have heard, nothing specific or first hand. Forums can be frustrating because things just get repeated by people who want to sound smart, but so very few actually "get their hands dirty" and figure out the actual problem.
I've thought about posting my finding on those forums, but I'm sure I'll just get beat up and people telling me I'm crazy and that's not the problem. It will be like the air spring replacement all over again, so I'll just keep it to myself and my fellow Garage Journal peeps, all five of them......
Scott, I'm not sure it's from actual movement because the tanks are pretty well mounted where there is not much movement at all and the flange is screwed directly to the floor of the coach. Also, our coach being of the semi-monocoque construction, there is not any movement in the floors and walls as there are on say a coach build using the hung wall design. I have no proof, but it could have been cracked during installation for all I know and being such a short piece of pipe, from the flange to the top of the black tank is only about 8 inches, it could be that there just isn't enough space for any movement.

It's hard to say, but I will be cognitive of the position and how tight I tighten it upon assembly.
You know the old saying Ryan, if the only tool you have is a hammer, everything is a nail. Well, I've got the TIG welder, so why not? Plus I haven't welded in a few weeks and needed the practice.