If you are lucky, this round "hole" is where the connection is made for the holding tank vent. Again, if you are very lucky, there may be a barbed fitting around this "hole" and all that you need to do is find the vent hose, and reconnect it to the barb.
The more likely scenario is that you broke the barb for the vent fitting off of your holding tank. The other scenario is that you have cracked the tank.
Here is a diagram (thanks to West Marine) of what the holding tank plumbing should
I believe that your "hole" is at, or near, 5 in the diagram.
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