We went through the same thing
Only our 9 1/2 yr old went from normal to horrible in 5 days. Her eyesight went soon after her unsteadiness, then she could no longer lay down - I had to physically lay her down. She could get up, she would eat the vet's special food, and she could potty, but that was about it. The morning we found her stuck in a corner on our screened deck was the day we decided to put her down. It was like she knew - she actually jumped into the car after not being able to walk for 4 days. Then, when the emergency room took her in the back to weigh her, she wagged her nub (which she hadn't done for days). They told us that many times dogs have a rush of adrenaline when they know it's time. I'm crying as I type this, and we miss her terribly, but it was the right thing to do. We now have another puppy, and things are getting back to normal, although she's always with us.
You'll know when the time is right. Our vet wanted to do a bunch of tests, but as rapidly as she declined, we had the feeling it was only for the money. We don't use that vet any more. Many times medical tests are necessary, but there can be times when it's just as caring to let them go.