Go to any store that sells kerosene heaters and fuel. You will usually see kerosene fuel dryer there AND it comes in beautiful scents like pine or whatever. You don't need much additive, only 1 oz. per 10 gal of fuel.
However, to be honest, the additive is mainly alcohol. Alcohol has the unique property that it absorbs water AND is soluble in oil based fuels such as gasoline, kerosene, diesel, etc. So I would go to Wally World and pick any fuel dryer you see on the shelf. Use the proportions on the bottle. You may have to give it several doses to get rid of the water but it will work. I would just add it to the fresh fuel. That way you can do it once and each time you add fresh fuel, you will remove a little more water.
If you want the fresh smell of pine trees every time you fire up your heater, use the kerosene dryer made for that purpose. I might add that the scent is subtle. No one will ask if you just came from the forest but they will notice the kerosene smell