It is generally not recommended to leave your black tank open when you put your RV into storage. The black tank holds the wastewater from your RV's toilet, and leaving it open can cause the liquid to evaporate, leaving behind solid waste that can stick to the tank's walls and create unpleasant odours. This can also lead to the development of bacteria and other harmful organisms, which can be difficult to remove and cause damage to the tank's components. Instead, it is recommended to keep the black tank valve closed and add a special treatment product designed to break down waste and control odours. This will help ensure that the tank stays clean and in good condition, ready for use when you take your RV out of storage.