'Polluted' Fresh Water Tank

We are new to motorhomes and have just become the owners of a '97 Autosleeper Symphony on a Peugeot Boxer base.  The dealer filled the fresh tank and we spent the first weekend away trying out our new hobby.  Although we weren't using the fresh tank water for drinking and cooking purposes, everything was fine with clear water in the taps and WC flush.  Towards the end of the trip, on drawing through the sink tap there was an unpleasant smell of white spirit and cloudy water with an oily sheen.  We are now guessing a few drops of diesel have accidently been put in the fresh tank at some point in the past, whether recently or long ago.  What are other people's experiences and how do we clean up the mess?  Many thanks for your thoughts..