OK, so my grey tank is probably the most sanitized grey tank around now but still have the problem. I started tracing it further and have found it to actually be in the cold water supply line. I am connected to city water but intend to disconnect and sanitize the water lines. I discovered this by opening the low point drain located in the basement. Both hot and cold low point drains there but only the cold had odor. So this is actually good news. Thanks to all for the suggestions but if you find this problem in your rig, check the fresh water supply. Only thing I can think of is that there is some water in the drain tube that doesn't get cycled out with regular use.