My bathroom sink also drains into the black tank. I found this out thanks to the MOC.
Steve,
I found out what tank the sink drains into while I was dumping the black tank. When the tank is finished dumpging, go back in and turn on your cold water in the bathroom sink. If the tank starts to drain again, your is in the black tank. If not, your dumps into the grey. I heard that Keystone changed the sink drain to empty into the gray tank now instead of the black. My bet is yours is in the grey tank.
2004 Chevy Silverado LT 2500HD CrewCab Duramax
2004 Montana 3400RL