Welcome to the forum. You are correct about the valve on the passenger side. It stays closed until you are ready to drain the black tank. After the black tank is empty, close the passenger valve first. That will allow the crossover pipe underneath a chance to drain out. That valve is there to prevent accidental spills on the highway should you damage that crossover pipe. When at a campsite, you should keep the valves closed until ready to dump. You need enough liquid to help move any solids out of the tank when you dump plus you don't want any campground sewer gas from entering you motor home.
Paul
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