As others have said, some rigs do plumb the bathroom sink water into the black water holding tank.
I'm thinking that your rig has a separate holding tank for the 1/2 bathroom sink & toilet, is that correct?
In our rig, all black and gray water go to their respective tanks. If the gray tank overflows, we'll see it in the shower stall. If the black tank fills up...well...you just see it when looking down into the john.
Having a Winnebago product here is definitely an advantage because the technical docs are all online, as posted by craig7h. I really like our rig, but as one who does all of my own maintenance, upkeep and upgrades on the rig, it would be REALLY NICE to have access to all of the documents like Winnebago provides.
~Rick