Welcome to the Forums (or at least, to your first post).
I can't say I have seen a thread with your specific problem. Another picture showing the "big picture" layout of your bathroom might be helpful.
At a glance at your picture, though it looks like your bathroom was assembled somewhat similarly to my '73 GT. In my case, the black tank sits on the floor and has a piece of plywood sitting on top of it to firm up the upper surface. They then lay a molded ABS plastic "cover" over the tank and plywood and attach that to the wall and cabinetry. This is a marginal design in my opinion, because all of the user's weight is supported by the tank alone.
So if yours is built as described above, I would guess that the tank has been smushed (technical term) to the point that the molded plastic has pulled out of the fasteners connecting it to the cabinetry. The tank may be permanently deformed (and possibly cracked as well).
So to fix this, you may have to disassemble your bathroom fairly significantly (again, pics would help), as the tank is one of the first things to be put in place, and everything builds on top of that.