Re: The Geo Method / Black and Gray Water Holding Tank
The Calgon doesn't do anything about the sewage, but it increases the 'cleaning power' of the soap and water to help get the crud off the walls and floor of the tank. The Geo method seems to work for a lot of people who try it, and I don't think the guy who presented it gets anything from it, except perhaps pride.
You really don't want anything in your tanks when you are storing the RV. Before storing it, dump and flush the tanks really well. Store it empty. In fact, for long term storage, they make a grill which fits over your dump pipe so you can leave the valves open, but I haven't tried it. No problems so far just storing it empty.
If the chemicals you use are enzyme based, I have heard that heat kills the little buggers. If they are not enzyme based, they may or may not have a problem with long term storage; I'd be nervous about leaving anything in the tanks for a 'long time' (a month or more).