greener,
If I understand your question/comment, please remember that the plumbing on Montanas is NOT always the same configuration. There was a time that they actually plumbed the washer/dryer into the black tank.
To be specific, your unit is most likely plumbed differently than our early 2004 model, my w/d goes directly to my GRAY tank. If the tank is not open, you risk the chance of overflowing.