I just ran a test on my bath AND kitchen sinks using a bucket of clean water, opening the gray water drain valve, pouring the water into each sink separately, and checking the outside drain to be sure the water ran out. It all did. I am totally dumbfounded
that your MoHo is plumbed with the bath sink going into the black tank! Is there any chance a PO did some plumbing "work"
on your MoHo? If Aistream did that, it's totally inexcusable. I realize I'm not telling you anything you haven't already thought, but since most, if not all, of the Classics had the same bath set-up, i.e. sink in the bath with the commode up on a "platform", it just blows me away
your sink was piped into the black tank. I had to rebuild my commode platform several months ago, and you may know this, the commode is literally plumbed into a house-type 3" PVC pipe almost straight down into the black tank. The drain pipe for my sink goes along the side wall inside the vanity toward the rear of the coach until it goes down into the gray tank almost behind the bath.
Once you recover from this
, are you going to attempt to redo the drain from the sink? One other unpleasant thought...have you tested your kitchen sink to be sure it's not draining into the black as well? My sink and bath drains join inside the bath vanity, and then go into the gray tank.