If You have a really bad issue with algae or something else then peroxide is what is usually recommended followed by bleach to then sanitize... Bleach is your best and most effective item to use to keep the RV water system sanitized... I also would not leave water in the WH or fresh water tank when RV is stored between trips, etc.. i Cup of bleach with 1/2 tank of fresh water will allow a full sanitization of the entire system ( run thru all the taps, toilet, outside shower station, washer hookup, etc... Then Let this mix sit for 24 to 48 hours for best results followed by draining and flushing with fresh water