The reason I am asking Are they sure it is spinal. He is a large breed, could simply be an extreme case of Pano, I had 2 shepherds who behaved exactly the same way until they were 24 months old it was Pano. Has the dog been checked for Lyme Disease? This could also cause these symptoms. I find it hard to believe that such a severe spinal problem would have not had previous symptoms. Don't read too much into the overstimulation of the rear leg reflexes both pano and lyme cause this and they are not a death sentence. I don't know where you are located but these Doc's seem to be making alot of money off of you. Start googling Pano and ask the Doc. Also ask for a Lyme Disease test. If you bought him from a breeder contact the breeder and ask for a history, most breeders will tell you if you present them with such a sad story. If you bought him from a pet store your kind of out of luck there you can ask them which kennels they buy newfie's from and they should tell you if you don't put any blame on them.
I have a question. I am not a vet but have extensive experience in dogs, being a behaviorist and a rescuer. The dog was behaving fine then just stopped walking the next day? Is it both rear legs or one more than the other? The meds she is on will make her very very tired so it is hard to judge her mood, but try the best you can. Where did you get her?
This is tricky. I have no experience with spinal disorders. It might be useful to you to speak to one of the vets on the "Ask a Vet" forum. You will most likely get some good advice there. I believe there is a small charge for this, but I am sure it will be worth it.
As I say I have no experience and don't want to give you possibly inappropriate ideas, but depending upon the full diagnosis, and your vet's advice, it might be a thought to consider walking aids for dogs! If your dog becomes paralyzed in her legs, yet her organs function normally, and she is not suffering in any way, then it is just possible a walking aid might give her a bright new life. "dogstogo.com " might interest you if she is a suitable candidate. There are also other companies who manufacture these aids. You could google it. Many paralyzed dogs have been given more happy healthy years with these "dogs-on-wheels" aids.