In addition to what Susie suggested, I suggest you put into the mix of consideration something I happen to come across by accident. When it comes to rotti's, and other large canines they are also suseptable to problems of pinched nerves, the symptoms can look very much like seizures. Seizures don't always involve vomiting, drooling, shaking , spasming or lack of eye movement or rapid eye movement. about the only thing that every person and every animal with seizures has in common is their lack of ability to contact the world around them during an episode. You really need to get some xrays done to rule out disc deterioration,arthritis etc, and if such is the case There are meds that can help and a chiropractic vet can absolutly do wonders . I have heard of success stories with symptoms you are describing (from when I had to take my own mixed breed for accupuncture treatments that had wonderful out come) Especially if it is a pinched nerve. As I stated throw it into the mix of possibilities until you have more diffinitive proof either way.
We ALL wish for the best