Sorry about your bunny
I had a rabbit who had a brain tumor and my vet told me this was very common in rabbits, and causes seizures/strokes. I had my rabbit put to sleep so she wouldn't have to have more seizures, vet said she would have died within a few days anyway.
I know it's hard to make the choice to put them to sleep if they don't seem to be suffering...and a car ride/vet visit can be really traumatic for a rabbit...Sadly once they're like this they usually don't get better though