Since you are draining at a bottom port, you'll need to get rid of any trapped air within the bag.
If you have an extra port on top of the bag you can just run that to a above the water thru hull or use the drain thru hull with a check valve.
If you don't have an extra port, make your fill hose go above the highest point of the bag. At that point, 'T' off the hose and run the one end out a thru hull or connect to the drain line via a check valve. The 'T' must also be below the thru hull.
Hope this helps and makes sense.