I just did this. I poured 2 gallons of pink antifreeze per bag and ran pumps till they offloaded and pink came of ports on bottom of boat. I manipulated the bags to get most of the fluid out. I did the same for the bow bag, but used 4 gallons to make sure the center tank had enough pink antifreeze in it.

I went ahead and pulled my bags so I could clean them and deep clean the boat. Previous owners let some mildew grow and now was the time to clean it all up.

Wasnt very hard, just time consuming. Winterizing the engine was a lot quicker.