In 2006 while in Alaska a guy next to me with a 5th wheel had something similar happen with a loud bang. When I went over to see what happened the tank was bowed , hanging down, with the c channels that hold it up bent very badly. I later found out he was loosing water every time he moved so he installed a check valve so the water would not siphon out while traveling. He forgot to open it while filling and it expanded and broke the supports. After a trip to Walmart withe some FG and resin myself and another fellow were able to fix it for him. I mention this only because I can't see why this would just fail on it's own while parked without something else causing it.