If you want to put antifreeze into your fresh water tank, the neck of a large drink bottle fits the fill nozzle perfectly. I put antifreeze in our fresh water tank to winterize. It takes about a gallon more than if I had an easy way to use the water pump to suck it out of the bottle.
I like to have antifreeze in our tank. The drain isn't on the bottom so some water will be left in it, which could freeze. I'm probably too conservative but it's worth the extra effort to me to avoid broken tubing or fittings.
2011 Sunseeker 3100
2016 Ford Edge Toad