I'm sorry, it's never an easy decision to make. Seizures in young dogs is usually caused by epilepsy or some environmental factor. However, in geriatric dogs it's more commonly caused by a tumor, particularly in cases where the seizures worsen gradually over time.
It comes down to a quality of life issue. From my understanding, the seizures themselves are not generally painful, but they can be frightening and disorienting. Treating with medication is always an option. Yes, seizure control medications can often make some dogs a little drowsy. However, it's not like the only thing she'll do while she's on the medication is sleep (and, quite honestly she's probably spending a fair amount of time napping and sunbathing as it is anyway with her advanced age). And some dogs don't get drowsy at all on the medication. If it were me, I would probably at least give it a try, maybe just a week or two, and see how she does on it. If you feel it makes her too sleepy to the point that she is missing out on the things she enjoys in life then you're back where you are now. But if it doesn't decrease her quality of life, then it could buy her some more quality time. When to make the decision to let her go really depends on what kind of quality of life she has between seizures and how long she seems disoriented and/or distressed after the seizure.