To get good answers you have to provide lots of detail and pictures if possible.
I am not familiar with your specific unit.
Are you sure its the black tank.
Most black tanks are directly below and attached to the toilet.
Take a flashlight and look down the hole.
If you remove the toilet, you may be able to determine the source of leak?
Is there any movement of the black water tank??
Possibly the vent line is leaking?
Usually the grey tank fills up before the black. (depending on the size of tanks)
I have never been close to filling the black tank on my Clipper, but have flooded the bath tub with the grey water a couple times.
Let us know what you find.