Keeping water tanks clean and fresh

number9

hi, I'm a complete newbie, having just bought first campervan (van 20yrs old!). My van is in decent condition for each but I'm not sure how much it has been used over last few years/how maintained.

How do I ensure that the fresh water tank is clean(ed) (approx 68l capacity) and likewise clean the grey water tank?

Water system links to carver water pump and propex heating system, kitchen sink tap and also to the bathroom shower/sink combi tap (whale). thanks.