Veterinarians will sometimes lie just to make money.
I'm not going to accuse your vet of doing this because I can't say for sure, I would be a little more skeptical though.
You said that it's obvious that your dog isn't doing well, so what makes you think that she's doing well..?
And btw if your vet or anyone says that she is suffering and needs to be put down, I just want to say this now in case I'm not here and it comes to that, please do have her put down. It's a very humane process, the dog doesn't feel pain it just thinks it's falling asleep.
Trust me it's less depressing to have them put to sleep then to watch them suffer to death.
This is something I didn't understand when I was younger and our cat had to be put to sleep. I just thought the vet was heartless, but that's not it at all.