Most water carries micro organism. When that water is retained inside of a enclosed space (like a motorhome fresh water tank) bacteria can multiply rapidly. This turns into even worse in warm temperatures. Legionella is amongst the principal considerations, but so is E-coli and a number of other other types https://trentonte0j1.jts-blog.com/30363482/top-guidelines-of-how-to-clean-water-tanks-in-al-bukayriyah