Dear Kris, I am so very sorry about your dog!
Did your vet examine your pets brain for a brain tumor? Brain tumors can be the cause of some seizures. Other possibilities might include: toxin ingestion, a microscopic cancerous disease, and any microscopic disorder that could not be identified without cytology and pathology via microscopic diagnosis.
It might help to contact your dog's breeder, if still available, for more information.
Thank you so much for the info.