we have always used. RV antifreeze. This year we opened faucets and drain lines Heading to Florida from Mo in Nov.. LAST SPRING, ON OUR WAY BACK FROM FL I PUt A bag oF ICE IN THE LAST MORING OUT. WAs sURPRISED. AT THHE. GUNK IN BLK TANK EVEVn AFTER Sprayying it out good. This trick worked well. Also after 43 years RV'inng foud A product that lubricates "pull handles on tanks". Could save some real headaches if handle were to break. RV dealer...tank gasget lubricant.