The water in the water tank of a caravan, camper and boat is almost stagnant. Depending on the size of the water tank, you will typically have several days to use the water in it. The water in the tank can't be kept cool such as water at home but easily reaches temperatures of 20 to 30 degrees a day. This means that the water is predisposed to development of algae and bacteria.
Therefore, the water tank must be cleaned thoroughly at least once a year. Most people do that as a part of "Spring cleaning". That way you can be sure of having cleaned the algae and bacteria before the new camping season starts.
It's not very difficult to clean the water tank and water hoses. There are multiple products for that on the market. Here at ScandiHills.com you will find the most popular ones.