As I understand your question, you can install a bypass valve if you have the
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If I understand, you can split the gray water from the holding tank if you have the space for the plumbing. Simply stated, you have to disconnect your gray plumbing from your holding tank. Install a "Y", with a directional valve, or two gray dump valves. Probably easiest with the two gray dump valves. Connect one of the dump valves to the holding tank. Add a dump connector to the second gray dump valve. Ideally, the gray dump valves will be higher than your holding tank. This will prevent back flow if you open the gray dump valve to dump connector without closing the gray dump valve to the black tank first.
Wayne