The usual spot for holding tank vents is a few inches below the toe rail on the freeboard. It is important to have the vent holes near the tank so the vent hose is kept short. When we replaced our tank we added a second air vent to the tank as the more air in there the less stink is created. We use the KO bacterial tank treatment and have never smelled the tank while sitting in the cockpit.
Midships is probably not the best location for the vent(s) for the reasons you mentioned.
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