Obviously you cannot use the water out of this holding tank because the Antifreeze that is used in cars is highly toxic and should never be used to winterize an RV.
The fresh water system on your RV needs to be cleaned out. My suggestion to you is if you know who manufactured the antifreeze is to contact the manufacturer and see what suggestions they have for you to get rid of the antifreeze in your fresh water system.
You may also want to contact the National Poison Control Center to see if they have any suggestions for you.
Everything-About-RVing.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to amazon.com.