Thanks for the additional information. I really appreciate it!
From your description of the tremors and the fact your vet has you treating with Diazepam, I suspect that your dog is having focal seizures.
Normally when we think of seizures, we think of the grand mal type, where the whole body spasms and the dog loses consciousness. However, there is another kind, called a petite mal where only one muscle group is affected or the dog exhibits some kind of strange behavior. Sometimes this can look like just muscles shaking, or the dog can be doing fly-biting (snapping at invisible insects) or they can just have overall tremors. I suspect this is what is going on with your dog.
The cause of these kinds of seizures can be anything from ingestion of a toxin to low blood sugar (you could try rubbing a little Karo or pancake syrup on her gums to help rectify this if it's the issue) to some kind of organic brain disease or organ disease. Obviously, there's no way to know for sure which is going on with your dog without being able to see her, but since she's an older dog who seems to be having increasing frequency to these episodes, I suspect there's a neuro issue here. Whether it's something like a tumor, or just a gradual degeneration because of age is impossible to know.
Unfortunately often the cause of the seizure isn't found. They call that a "seizure of unknown eitology", which essentially means that yes, the dog had a seizure, but we don't really know why.
I think at this point, you might want to talk to your vet about getting her on some Phenobarbital...it's a medication given every day that helps stop these kinds of tremors. If he feels that her tremors aren't bad enough to warrant it, then you can absolutely continue to use the Diazepam. It's very safe and frequently used for these kinds of issues.
I hope this helps!!