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Hi my name is Robert.....
Will the other grey tank drain without the black tank valve opened up?
Click HERE and see a few pages down, it will show the plumbing for the valves, that piping should come in to the center for the gray water 2 or smaller pipe,it goes to the center of the flange between the other valves so with only that valve open it should be coming out unless they plumbed it before the flange for the black tank, if that was the case the black tank valve would need to be opened up. Each valve has a gate that moves back and forth. Robert
Look at the plumbing diagram I sent and make sure it is plumbed so the smaller pipe is in the center of the two others, if they had that pipe plumbed prior to the black tank valve then that valve would need to be open to drain the tank. Follow that piping back and make sure there is not another valve as well. Sorry the company charges an added fee for a phone service and I will be in and out this AM.... I am hoping the diagram might shed some light on what is going on here... I will check back this am to see if you have responded....