OP, your tile lists a 2008 trailer; my husband and I own a 2007 MH, very close in age to yours. If Airstream plumbed each in a similar way, you may have a cracked vent line which has allowed some black water to seep out, leading to odor.
My husband and I suffered from the infamous plumbing defects of that era, which caused our gray tank to shear away entirely from its three lines, and the common vent line to shear off the black tank. You can read about our vent line DIY repair job here
, and our gray water fix in Part 1
, Part 2
, and Part 3
of a blog post series I wrote.
Note that a vent line crack, if present, may be subtle and may not fully activate until your rig is on the road and vibrating around. We helped some friends to re-plumb their rig this past weekend (they have the same model and year that we do), and I had to look carefully to confirm that their vent line was, in fact, partially sheared, just as ours had been.
I don't know if this build issue extended to any trailers, or was restricted to just Airstream's Class B vans. I'm curious to find out.