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Usually, the black and grey tanks are side by side, and drain into a common 3" drain line. Each tank has its own valve located at the tank outlet. The galley tank is usually a separate 1-1/2" line that tees into the 3" line near the outlet. Having said that, I acknowledge that there are many variations in the construction of Montanas, but in all cases, the black tank definitely should be 3".
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