My Georgetown Owner's manual says to put automotive (glycol) antifreeze in the black and gray tanks. I figure if they're drained, there's enough air space that damage to the tank is unlikely in our climate (not usually below freezing during daylight hours).
I'm a bit surpised at FR's recommendation, as I didn't think ethylene glycol was a good product to dump in the sewer or, particularly, in a septic system.
Frank Damp -Anacortes, WA,(DW- Eileen)
ex-pat Brits (1968) and now ex-RVers, as of 08 Dec 14.