There’s no reason to put RV antifreeze in your fresh water holding tank.
Just make sure to empty using the drain plug of the tank.
To put sanitizer into the tank, using a funnel, pour some into the hose, then connect the water hose to the on-board tank inlet connector of your motorhome and turn on the water.
Ken and Terry
2010 Georgetown 330TS being pushed by our
2017 GMC Terrain toad (4 wheels down)