Put in a pick-up tube from the top of the tank. Run the outlet from that tube to a Y-valve and then run one side of the Y to a pump and then to the thru-hull. We have our tank set up that way. To pump out you set the Y-valve appropriately, open the seacock and pump the pump. It normally takes 50-60 strokes or less to empty our tank. When you finish close the seacock. A tank with top fill and discharge CAN'T leak around a fitting because all the fittings are on the top of the tank, unless you completely fill the tank and keep pumping into it. The top of our tank is about 3' above the bottom of the head and it isn't a problem to pump from the head into the tank. With this setup you don't need a siphon break anywhere in the system unless the top of the tank, where the vent line is, is below the waterline. The vent acts as your siphon break. Actually the only siphon isk is that once you start the tank discharging through the thru-hull, you may set up a siphon from the tank to the thru-hull which will drain the tank effortlessly. In addition, I plumbed the head intake into the drain for the vanity sink. To flush I run the appropriate amount of water into the sink and then flush. A fresh water flush greatly reduces odor. You don't use that much fresh water either since you can control the amount of water used for each flush. Keep the sink drain seacock CLOSED, unless you are draining a lot of water from the sink. Furthermore, using a fresh water flush like I describe removes the need for a siphon break on the head inlet assuming you keep the sink drain seacock closed