I’ve used Starbrite many times and its what I recommend on this site. Here it is on Amazon: https://www.amazon.com/AMRS-31400R-6-Starbrite-Non-Toxic-Anti-Freeze-Restrictions/dp/B00NKV0QM8
I’ve also used Splash Rv and Marine Antifreeze, which I picked dup from Menards. That did the job too.
The most important thing is to get an antifreeze specifically made for RV plumbing use.