Originally Posted by Terra Nova
There is no reason for pumping out the top of the tank. And no need for a vented loop, as the sea cock would only be open during pumpout at sea.
There's a very good reason for putting the discharge fitting in the top of the tank, with a pickup tube inside to the bottom: When the discharge fitting is at the bottom of a tank, waste will rise in that hose to the level in the tank, keeping that hose full of waste to permeate. When the fitting is on the top, that line is inside the tank.
While there's no need for a vented loop between the toilet and the tank, safety
standards call for one in any line that connects to a below-waterline thru-hull. The thru-hull SHOULD only be open during pumpout, but humans are faillible...the loop protects the boat from human error. You can never go wrong erring on the side of caution.