There is no water tank ! just a boiler tank with boiler fluid in it ( antifreeze ) with a water "coil" of pipe inside to transfer heat to the water. You can't drain it or blow it out totally and must pump pink rv water system anti freeze to protect it from freezing ! That's not what you asked but I've never had "stale water" problems .. just use it or run water through it. You could run bleach through the lines but be sure to not let it set its not good for the coils or so I've been told.
To properly winterize you must use pink . If any water remains in the coils and breaks it. You risk a $ 8000. ? Repair if you don't.