Recently I was at a Walmart (and no, this is not a post about parking there) and was buying chemicals for the black water tank when another customer (an RVer also, I assume) asked me what I used to sanitizes my tanks. At first I thought he meant the fresh water tank, but soon discovered he was talking about the gray and black water tanks. He told me that about once a month (and more often in hot weather) he fills both tanks to capacity with water (after dumping them) and then adds bleach. He lets that stand for a day or so and then dumps and rinses the tanks. All we have ever done is run water through the gray water tank and back wash the black water tank. We do keep the drop in chemical packets in the black water tank when in use, but we have never done the bleach treatment. Is this something we should be doing? Does anyone else "sanitize" their gray and black water tanks?