I winterized a couple days ago and wanted to add something to QBhoy and AllDodge's comments. When removing the plastic drain plugs, one disintegrated in my hand.. It of course was the hardest one to get to so I had fun with a screw extractor getting the plastic threads out. The previous year it was winterized by the marina and I don't think they use anti-freeze. This year I first dumped the water from all 5 plugs, poked in the drain holes (no rust at all but it's in fresh water) and after the large water volume rushed out I let drip for an hour. The next day I was undecided if I should do the antifreeze so I pulled the 4 old drain plugs and the brand new $7 one. I was very concerned how many drips added up over night and poured out of each drain fitting. I don't believe it would crack a block but it would be enough to easily take out a plastic drain plug and the 1" of fitting above it.
Because of that I carefully read the RV antifreeze label and purchased two jugs. Poured some down the raw water intake, then enough into the circulating pump hose so that it came out the thermostat housing. I checked the plugs and found the two manifold plugs didn't receive any antifreeze because the flow balancing balls won't open with that tiny pressure. I then removed the two lower manifold inlet hoses and poured antifreeze in that way. All 5 drain ports flowed pink.
Later I saw that despite the bottles looking almost identical, both stating -50F and containing propylene, the second one I grabbed also contained alcohol! So I then drained all of that out to avoid the unlikely chance that 10% alcohol would possibly attack rubber components. At least any remaining water was diluted with antifreeze before it was all flushed.
