[micacrumb]

Hi Just joined GSowners and saw your question.
If you decide to try and fix it yourself because the factory won't cover it, you will need to drill small holes at each end of the crack to stop it from growing. I've used hot glue in the crack. I first make a v in the crack with the hot tip of the glue gun and fill it with the hot glue. I've done this in the past on our old "78"class C on the gray and black tanks which were cracked by the previous owners by freezing. They were the white material just like the water tank.
Hopes this helps