The vent indeed goes into a stanchion on the port side of the boat. You'll see a hole in the inboard side of the stanchion. It's an inferior vent arrangement. (As Peggie H reminded me on another board..... )
If that vent is blocked, you can't pump into the tank.
On my boat I had almost the same problem. If I tried direct overboard
discharge I could not empty the head
. It came down to the previous owner's idea of installing a Y valve was faulty. Either the head was connected to the holding tank or the discharge thru hull was connected to the holding tank. (Bottom of Y went to tank)
Since there's no reason really for direct discharge here I just removed the Y valve completely.