I have similar questions about venting a holding tank. I'm in the final stages of a new holding tank installation; my tank was built with two vents but my mechanic has very different ideas than I do about the location and number of vents ... I would welcome some advice to resolve our impasse:
* Specifically, he wants to plug up the aft vent on the tank rather than go forward with two vents.
* As for the forward vent through-hull, he's concerned about siphoning, so he wants to run the vent hose in a loop above the through-hull before connecting rather than doing a straight run from the tank opening to the through-hull.
* In addition, he's inclined to locate the single vent through-hull on the side of the main cabin to reduce the risk of siphoning.
* As for me, I've been inclined all along to think there should be TWO vents, to run the vent hose in a straight path to the through-hull, and to locate them just below the toerail.
I appreciate any advice people may have to offer--