In my experience (not a vet, cat sitter of 15+ years) the yowling is probably from the deafness, and to a lesser extent, the dementia.

Has her activity level changed?

Does she exhibit any signs of physical pain or distress?

Is she grooming herself?

Has your vet suggested anything like Zofran or Cerenia for the nausea?

How's her appetite? Does she stay sufficiently hydrated? (drinks enough water)

Is she eliminating and urinating normally? Does the output look normal.

What behavioral meds have you tried?

If you have the means and it won't unduly stress out your cat, then the bloodwork could help figure out what's going on with her.

While many say 17 years is a long life for a cat (it is) many of my own cats and those in my care have reached 20 and beyond. If your cat has an issue that might be a be easily remedied and allows her to have some more happy and pain-free time, then why not look a little longer for non-heroic options before considering saying goodbye?

My 2¢.