I would also say 2-3 gallons. Why risk it becoming a potty cracker like substance, some of the water will evaporate, so better to have a little extra.
I use very little chemicals in the winter, more when it gets warmer out. The smell is only bad once it is above about 70F outside.
Another tip that I got from my RV salesman was not to add chemicals first. That is because there is a bend in the piping, and then the valve - all below the height of the tank. So if you where to add say 1 cup of chemicals first, then add a gallon of water, the chemical will end up next to the valve, and none would stay in the tank.
Fred.
