Sweeten your RV water tank periodically by flushing with 1 cup Baking Soda (Bicarbonate of Soda) dissolved in 1 gallon warm water. Drain and flush the tank before refilling (do not use Baking Soda in your drinking water). The Baking Soda will eliminate stale odors and help remove mineral build-up that makes water taste off.
There is also a product available in many countries called Milton that is primarily used to sterilize babies bottles. If you follow the instructions on the bottle is it very effective at sterilizing and deodorizing water tanks in recreational vehicles, caravans and motor homes.