How do you know when to say good-bye?

Shiloh took a turn for the worse last night, and was up and vomiting throughout most of the night. Just his saliva, no food. When we did finally get to sleep the thunderstorms rolled through, more vomiting, which woke up the puppy, who had to potty. Shiloh and I both were limp rags through today. I am stumbling-exhausted, fuzzy-thinking.
I called our vet and updated her and we had "the talk" about it being up to me now about making the quality of life decision. I'm not convinced Shiloh is ready to go, but on the other hand I don't see much fight in him either.
He can eat, he can poop but is that quality of life? He is deaf, his rear legs give way and my active OCD barky dog is just a shadow of his former self. He does have some moments such as his nightly teddy bear obsessive chat, but most of his days are spend in a deep sleep or just wandering aimlessly in the yard.
We can still up his meds 2 more times, and the vet said there is some other medicine we can add to hopefully help bring him back. We will try those things of course.
But - if they don't work - he is so young. How do you know when it's right or fair to let a beautiful young dog cross the Bridge? Is he suffering right now? It is very hard to tell.
I just hope that Shiloh gives me some sort of sign either way. I'll mortgage the house if he wants to live. I just need to know what the right thing is to do.