what holds black water tank in place

[tw64]

I purchased a 1964 Tradewind recently and of course it needs a little TLC. The black water tank had dropped almost 4", and so did the toilet. I removed the toilet and the underbelly at the rear bumper and then found an access panel at the bottom of the gate valve. It looks like the tank was built in a tin box, suspened some how between the frame. The "tin" seems to have rotted away leaving the tank in a dropped position. Also the aluminum cross brace( you see it when the bumper cover is raised) has dropped down with it. From what has been posted before this tank dosen't protrued above the floor. Any suggestions........