Clavamox is an antibiotic.
Torbutrol is a pain reliever.
Prednisone is a steroid that helps the cough.
Panacur is a deworming medication, might not be related to the cough.
There are a few causes for a dry cough. The first and most obvious is that the dog has a collapsing trachea. The trachea works like a straw and, if it collapses, it's like trying to suck air through a collapsed straw. This can cause coughing.
The second cause that I can think of is that the dog has a virus like kennel cough. Kennel cough usually improves with antibiotics but it can take a few courses.
The third cause might be a heart condition that has not yet been diagnosed. If the blood is not pumping correctly in the heart, it can cause some coughing.
I suggest that you give the vet a call and discuss some of these other options with them. See if any of these are a possibility (based on exam findings.)
Let me know if you still have questions.