Our first question is, how often do you use your caravan? The chlorine in normal tap water will kill off most bugs. So if you are regularly filling up your tank from tap and driving around (which will dislodge and treat nasties) your tank will be effectively self-cleaning and require no further care. But, left to their own devices, a lack of use of your RV will allow bacteria to grow in your tanks if sitting unused.
If you only use your caravan during the caravan season (school holidays, for example) store her with tanks bone dry, clean the tank and treat the first fill of water before use.
That said, you can never certify your tank is actually completely full or dry so a good water treatment regimen for your tank will help prevent nasties. There are tank sanitiser products out there to clean your tank but simply using lemon juice flush is our organic and natural cleaning tip.