I would try Anxietrex. I usually like to start with a half dose twice daily on the breakfast and supper meals; however, it is important to give it at a time away from the medications so the body has time to focus on it for a while before being suppressed by the drugs. You can give it without food if it is easier, in which case you can syringe it into the mouth. (You can add it to the water bowl but then the whole dose is usually not consumed.) In addition I would also supplement with probiotics because the intestinal bacteria are damaged by strong medications. OMEGA 3,6,9 is helpful in reducing inflammation and nourishing the nervous system.
What do you feed your dog, give as treats, kind of water, other medications? What age did the seizures begin? How is liver and kidney function? Has allergy testing been done?
I would also recommend an evaluation by a veterinary chiropractor and acupuncturist. Pressure on the nerves can contribute to seizures.