RV antifreeze isn't exactly non-flammable. At high enough temperatures it will burn, and vapors/mists can be ignited, even when mixed with water. Regular automotive antifreeze (ethylene glycol) is slightly more flammable than RV antifreeze (propylene glycol). Read the MSDS.
Unlike automotive antifreeze, RV antifreeze, doesn't contain as many corrosion inhibitors. Also, both oxidize when exposed to heat and air (plenty of both in this application) and become corrosive.
"...biological fouling also occurs. Once bacterial slime starts, the corrosion rate of the system increases. Maintenance of systems using glycol solution includes regular monitoring of freeze protection, pH, specific gravity, inhibitor level, color and biological contamination."
I would not even try to use any type of glycol based antifreeze in a water table.