There seems to be two ways they plumb them
One the bathroom lav (Sink) goes into black, The other Gray. There are arguments for both methods but I very strongly suspect the first is the better way.
here is why:
What makes "Black" water as opposed to gray or wash:
Fecal matter
And after you wipe your... self
you was in the bathroom lav, so if any of the Fecal matter got on your hands that's the sink it gets washes off in. (end arguemnt)
oh what makes the differnce between wash and gray: Food. Gray is wash water with food in it, This is why some RV's have 3 tanks, in my coach the main bath lav's and shower go to Wash, I assume the washing machine would if I had one but not sure.. (Do not have one) Half bath and kitchen to Gray, Toilets (both) to black.
And Coffee (that's brown water) to ME!!!!