This is a great thread, so to revive it I'll add this for un-winterizing your fresh water system:
Last year it seemed like I still had traces of antifreeze in the fresh water system until August, so this is what I did yesterday to make sure it was fully purged.
Get a barbed female garden hose fitting that fits your freshwater plumbing tubing (Mine is 5/8")
Remove the tubing from the output side of your freshwater pump, and insert the garden fitting.
Put a shutoff valve on the end of your garden hose
Connect the hose to the fitting you put on the supply line in the boat.
Open all faucets, and slowly open the valve to the desired flow and let it run until all traces of antifreeze have been flushed out. I ran it for 30-45 minutes.
This eliminated allot of wear & tear on the fresh water pump, and ensured that the system was clean.
For the water tanks, I disconnected the supply line in the bilge, and filled and flushed the tanks several times.