Lots of ways to skin a cat and my way sometimes gets criticized, but here goes.
I dont like to take chances, so I drain my fresh water tank, open the low point drains. Add 10 gallons of -50 RV antifreeze to the Fresh water tank and then cycle everything thu the coach until I see pink ...job done...all the plumbing and drains are winterized to -50 .... ( dont forget to cycle the washing machine if used ) now for sure now there are no low points hold water and the likes there of to worry about freezing and breaking
The critics of this process say the down side is the taste of the water afterwards.
That said, when Im ready to hit the road, I drain the RV coolant from the freshwater tank fill it with fresh water add 2- cups of household bleach and cycle it thru...... job done.
I dont worry about taste of the water because I dont drink it, I have always used bottled drinking water .... that said I do smell a bit of bleach smell from the first fresh water fill afterwards when showering but after that no problemo and I just disinfected the water system at the same time.
Many of the critics of this process do use air to winterize and that have have Aqua-Hot units but yet they prefer to cycle RV coolant thu the Aqua-Hot to be safe ....if that if concerned about low points in the Aqua-Hot and it freezing? ..... why not the rest of the coach? ...perhaps maybe the co$t of replacing the Aqua-Hot unit and that is the determining factor.?
ps: I also go to my coach and cycle everything including the water supply @ 2-months ( if stored) to keep all the seals and the likes lubricated ( might be overkill but it's only 27 steps for me to walk to my coach in it's garage )
Again lots of way to do this and you will see many good methods posted here ..."pick your poison"