Gray water tanks tend to be stinky if they are not cleaned well. I would add so,e bleach to the tank and fill it half full then drive over a curvy rough road. That should take care of the snell.
3 or 4mwould be my7 choice. I have never had a 1 pounder over fill. Some times I use ones that are about half full. You kno0w that taking refilled bottles across state lines is illegal,
