I understand the problem with not enough grey water space. That's one plus about our Serrano. We have two grey water tanks. The galley is separate from the shower. Also aids in that the washing machine dumps to the galley tank. That gives us four valves to open to drain all of the tanks since our black tank is also on the passenger side and has a valve at the tank. We also open the main valve on the driver side before opening the passenger side valve so the flow is direct without any excess pressure build-up to line before it enters the slinky.
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