I would be looking for another vet right away! If your dog is managing to behave himself, in spite of all this, he must be an angel. For a vet to be this scared, I'm sure he's transmitting it to the dog. If your dog has never shown any reason he should be muzzled, this is disgusting.
Maggie's breeder once refused to take one of her dogs on a national talk show because they asked that he be brought on muzzled. She said there was no way she'd muzzle this dog. He was an Am/Can CH, had his CDX and was a registered therapy dog and she refused to feed into the paranoia and sensationalism surrounding the breed. She said it was an afront to her as a breeder and to the dog as an outstanding example of the breed.
Sorry, I don't even live in the States and can't give you a recommendation, but this really infuriated me. Do you have any friends with Rotties who you could ask about their vets? How about your dog's breeder? The local Rottie rescue might have some recommendations.