Black Water backing up in shower when driving.

After getting back this weekend I think I found the problem. Top of black water tank is sagging about 1/2 way, causing tank to only partially fill. The vent tube is in the center of the tank, so it is lowest point on the top of the tank. Any ideas on a fix beside replacing tank, like some type of spacer or center support? I need some way to push the top/ceiling of the tank back to original heigth.....