John, I don't use RV antifreeze in either of my trailers in the fresh-water systems. I've used it in the past, but it really isn't necessary. I blow out all of my fresh-water lines with a compressor and leave all of the faucets open prior to storage. It's important to allow the compressed air to blow until all of the lines run 'dry'. It's not necessary at all to have every last drop water out of the water heater. The one place I make the exception and pump anti-freeze is in the shower drain and drain pump in my Scamp
. There's no way I know of to ensure that I can get that blown out properly, but come spring, I don't have to worry about it being properly flushed out either.
I DO pour anti-freeze into all of the drains, though.
It's not the water freezing alone that causes problems for RV plumbing. It's water freezing in a confined space with no room for expansion that causes problems. A drop of water left somewhere will just turn into a drop of ice without causing problems as long as it has adequate room for expansion. The little bit of water left in your water heater won't hurt anything as long as it can expand that little bit it needs to when it freezes. While air does compress, I'd just as soon the lines be able to breath as necessary.