I think (not saying this is the case this time) that many fresh water tank problems come from folks over filling (leaving the hose in the fill opening and getting distracted.
Water pressure will make that tank balloon and can break supports and damage all sorts of things. If you don't have a fill adapter that is smaller than the opening (to allow air to escape faster than just the vent hole), you will have a pressured water tank. I got distracted filling mine this spring and was very glad I had the small fill hose on the end of the white hose. Water was able to come back out around it and not balloon the tank..
