The holding tank is usually connected to the head and can draw air through the pump and water out of the bowl of the head on most boats. The vent is necessary to keep the holding tank from building up pressure, but isn't the only source of "air" into the holding tank when pumping it out.
In fact, I generally fill the head bowl with water just before I pumpout, and you can hear often hear the bowl being drained, and will know that the entire tank has been evacuated when the head bowl is empty. The water in the bowl also helps "rinse" the lines and the holding tank, which can help prevent odor problems from beginning.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.
—Cpt. Mal Reynolds, Serenity (edited)
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Still—DON'T READ THAT POST AGAIN.