Water system sterilisation after winter lay up

Hi, new to caravaning and have had our caravan laid up for the winter months. Due to venture out for only our second time in the Easter holidays. When I bring it out of its winter hibination I was wondering if anyone can recommend the best way to sterilise our water system. Friends have said they just use Milton fluid, but I wanted to know if I should use purpose made products from the caravan shops. Anyone help ?