Such a hard, hard decision. Your last camping vacation sounds like it highlighted how limited her life has become, both at home and on trips. No one here will fault you for your decision, whether to keep the appointment on Thursday or push past it. We've lost 6 dogs and I can't say any of them "let us know" that their time had come, we had to decide what was the best and most loving for them, and all were much younger than sweet Ruby.
After Chase died in June I read the book Going Home, Finding Peace When Pets Die by Jon Katz which I found very helpful. One passage from the book particularly struck me and I'll quote it here. It was in the chapter titled, Making a Decision You Can Live With.
"Animals cannot talk to us, but I imagine if they could, they would say something like this:
'Speak for me. Help me to make the decisions that I cannot make. Do not ask me to tell you when it's time for me to go for that is beyond my simple province. I love you and trust you and I have depended on you all of my life to make decisions for me. Now, when I need you the most, do not fail me. Whatever you decide, I know it will be your best decision, and I wish you nothing but peace with it.'"
I wish you peace as well.