I never put RV antifreeze in my fresh water holding tank. It isn't poison but it does have an odor and a taste that is difficult to get rid of next season.
I open the drain valve on the final trip home for the season and the movement of the trailer ensures the holding tank is sufficiently empty.
A small amount of water in the bottom of the tank isn't a concern if it freezes as there will be sufficient room for expansion.
The concern is water lines and the pump which could split if water freezes in them.
I cut the discharge line from the fresh water holding tank and installed a ball valve and a "T" fitting in this line. Then I attached about four inches of clearing tubing to the "T" fitting, another ball valve and finally, about four feet of clear tubing.
Closing the ball valve on the "T" fitting line and opening the ball valve on the fresh water tank discharge line allows me to pump water from my fresh water holding tank.
Closing the valve on the discharge line and opening the valve on the "T" fitting allows me to pump water from a jug without putting it into my fresh water holding tank, a good option when boondocking
I also use this line to draw RV antifreeze through my lines, pump, facets etc. when I winterize my unit.
Minimal amount of RV antifreeze required and no RV antifreeze in my holding tank to flush out next season.
Don't forget to drain your hot water tank!