I always sanitize the water if it has not been used for a few weeks. Pseudomonas and other bacteria and molds can grow in stagnant water. Even highly purified water will have growth. It should be quick and easy. Add about 1/4 cup bleach for every 15 gallons of water, run the cold water faucets and allow to stand for several hours. Some people are more thorough and also due the hot water but that means draining the tank afterwards or flushing with a lot of fresh water.
I do try to be careful about where I fill up the tank. Water in national parks and national forests is tested. I do try to avoid well water from untested sources. I am comfortable using a Brita for final filtering of drinking water. After years of RV travel I have not had any issues except for hard versus soft water.
