I had a tank like that before. I placed kitchen cloths around the top, pour neat vinegar onto them and just kept them wet(with vinegar) and after 24hrs, I use a paint scrapper, the razor blade ones. ALL the gunk came off, even the cloudy stuff.
Also a friend(!!) used a bathroom limescale remover called Cillit Bang. Turned the tank upside down, sprayed the area with the cillit bang, waited 5 mins and every spec of gunk came right off. He then used a garden hose and gave it a really good rinse... He had that tank until very recently and had NO issues from using the product....