When you pump out at a station the huge suction will clear the air vent. You'll hear air going in. As others have said, station pumpout is not ambiguous. Those pumps could suck the chrome off a trailor hitch, to quote Willie Nelson in reference to a rodeo consort. I usually fill the tank with a hose first, maybe some detergent, just for the full colonic massage experience. (Last week when I pumped out nothing happened. Huh? Yeah, I had forgotten to change the valve settings.)
By the way, holding tanks are not for holding. They should be pumped out as soon as possible--a day or two at most after use. Storage of effluent is what makes the permeation and the smell.
Just about everybody adds Odorlos or similar to the holding tank to break down remaining stuff. That requires air, and a functioning vent hose. You're in lake water, but salt water boats find fresh-water flush equally important.
By the way, holding tanks are not for holding. They should be pumped out as soon as possible--a day or two at most after use. Storage of effluent is what makes the permeation and the smell.
Just about everybody adds Odorlos or similar to the holding tank to break down remaining stuff. That requires air, and a functioning vent hose. You're in lake water, but salt water boats find fresh-water flush equally important.