Fresh water tank drain.

While there's no real harm to having antifreeze in any of the tanks, if you drain them normally, there should be no need of keeping any antifreeze in them. Whatever miniscule amount of water remains on the bottom of the tank has plenty of room to expand as it freezes, and no fittings to fill and burst/split apart.

If you have any significant amount of water remaining in, say, the dump line between the tank and the dump valve, that could very well lead to trouble; but a quarter inch or whatever in the bottom of a tank is not a concern.