Gosh. I'm so sorry for you and your rabbit that you are having such bad luck with your vets. I agree with Majorv, I can't believe the vet didn't just look up rabbit syphilis if he had never heard of it. He's not even trying.
I found an old post about someone who had a rabbit that had syphilis, thought you might like to read it. They said their vet prescribed Convenia, which is a single shot that lasts 7-10 days and seems much easier than penicillin (she had 9 rabbits). But she doesn't say at the end of the post if it worked or not.
http://forums.rabbitrehome.org.uk/showthread.php?127294-Rabbit-Warts-UD-off-to-the-vets-now/page4
I think vets that don't know much about rabbits just prescribe Baytril because its used on cats and dogs, they have lots of it, and it was the first (maybe only) antibiotic licenced for use in rabbits so its just a no brainer easy thing for them to prescribe without having to look anything up.
I found an old post about someone who had a rabbit that had syphilis, thought you might like to read it. They said their vet prescribed Convenia, which is a single shot that lasts 7-10 days and seems much easier than penicillin (she had 9 rabbits). But she doesn't say at the end of the post if it worked or not.
http://forums.rabbitrehome.org.uk/showthread.php?127294-Rabbit-Warts-UD-off-to-the-vets-now/page4
I think vets that don't know much about rabbits just prescribe Baytril because its used on cats and dogs, they have lots of it, and it was the first (maybe only) antibiotic licenced for use in rabbits so its just a no brainer easy thing for them to prescribe without having to look anything up.