Drontal is good for hooks, round and tapeworms. And Revolution is great for hooks and roundworms and fleas. I only use Revolution on my cats, I use it on them about every 3 months since they are strictly indoor cats. I would just use the Revolution if your treating for roundworms. Use the...
Have you had the vet check for a urinary infection? Sometimes those can manifest with varying symptoms, like urinating or defecating outside the box. I would be tempted to take the covers off the boxes too and try that. Also, you may want to find another vet. At least to get another opinion...