Few will run into this problem, but this may help someone. 2014 Lounge EXT
The lavatory leaked; sometimes water would collect in a small puddle in the aisle. (not in the bathroom)
I then noticed wet spots under the van near the bathroom.
The lavatory uses a Hepvo trap, not a p-trap or s-trap.
Sorry, I didn't think to take pictures during the discovery phase.
The trap has a swivel nut on top and is meant to install directly to the sink drain body.
For some reason, Airstream added a bushing and swivel nut between the drain and trap.
Although hard to contemplate, the swivel nut had sheared in half and that's where the leak was.
The fracture wasn't readily visible.
Now this would seem to be an easy plumbing repair, but it's not.
There is a series of fittings in the cramped space and no slip joint or union. You would expect a slip joint in a waste assembly.
There is not enough room to insert the replacement screwed or glued connection parts. I tried a few things and contemplated many more.
In the end, after making me suffer, the gods smiled on me. The short remainder of the sheared swivel nut actually had enough thread left to connect with the drain body.
There doesn't seem to be any leak and the assembly seems stout enough to endure.
The leak was a result of both dubious design and QC by Airstream. The waste assembly design could be re-jiggered, but the space is so limited that install really needs to be done during initial manufacture..