Bathroom flood - What did I do wrong?

dougrainer wrote:

Dutch_12078 wrote:
Bathroom sinks are sometimes plumbed to drain into the black tank, likely to help balance the capacities. My last two motorhomes have been plumbed that way. The RVIA standards permit one device besides the toilet to drain into the black tank, but the shower is not normally used because of the higher backup risk.

The ONLY reason that any Gray source may go to a Black tank is because the floorplan design of the RV keeps that particular Gray source from going to the Gray tank. Doug

With all due respect, Doug, that's not always the case. The bathroom sink drain in my current Coachmen for instance, has an elbow that's just above the grey tank that directs the drain to the black tank about a 18" away. The drain could very easily, if not more easily, have gone straight into the gray tank.