It’s a massive job. The tank is bonded into the hull before any of the furniture is dropped-in so you have to start cutting it up to get it out.
I would recommend that you get some Marine 16 treatment and see if that sorts it, and if not get the tank emptied and flushed through.
Try this stuff. It works well:- http://www.seamarknunn.com/acatalog/marine-16-diesel-bug-treatment-100ml-4240.html?gclid=CPGX0PKhhMoCFSoUwwodoJEDew
