RV antifreeze vs Regular glycol antifreeze, what's the difference?

There are two types of glycol in common use these days. One is ethylene glycol typically used for cooling internal combustion engines and propylene glycol used for freeze protection in hydronic heating systems and rv potable water systems for winter storage. Ethylene glycol is highly toxic and is typically more expensive while propylene glycol is non toxic. Either will work fine for freeze protection but if your system is "open" I would suggest propylene glycol to prevent any accidental poisoning. The only true way to tell how well you're system is protected from freezing is by use of a refractometer that will tell you the percentage of glycol to water and to what temperature the mixture will protect.