I found it. Like another poster, my W/D unit drain hose drapes into the ABS drain like at home, but there was nothing sealing it! So, a nice big gaping hole straight to the smelly gray tank. Doesn't take much negative pressure to pull that awful smell into the coach. The good news is that I was able to get to it through the access panel in the closet and fix it. I used a 2" rubber cap from Ace, cut a hole in the center slightly smaller than the hose, pushed the hose through, and fastened it to the standpipe with a hose clamp. Perfect seal, no smell.