Black Tank Leaks

I have a 2009 Big Country and when the black tank gets full it overflows and leaks into the garage compartment. Is there an overflow valve nat the tank that prevents the camode from overflowing or do I have a more serious problem

Phil, welcome to posting on the forum.
If you think that it only leaks when it is full, take a look at where the commode drain pipe enters the black tank. You should also see where the vent pipe enters the tank. Something may have come loose or the seal is failing.
You may be able to seal it somehow with some good caulk. Or perhaps you may need to cut the pipe and redo the plumbing.

Peace
Dave