Is it time to euthanize our black lab

Question
We have a 9 yr old spayed female lab who was diagnosed with Crushing's disease last April.  She exhibited few of the symptoms of the disease, but she had had blood work before we were going to remove multiple fatty tumors she has on her abdomen/rib cage area.  He phospholipotase (sp?) was extremely high.  She was placed on Lysodren at that time and takes it once/week  since then.
About 3 weeks ago she became less energetic to the point that since last weekend she stopped eating, stopped retrieving the newspaper in the morning (an event that she NEVER has missed in 9 years!!).  We have taken her to our regular vet...blood work, lymph node aspiration both have showed nothing definitive other than it is not obvious lymphoma.  She has more and more crustiness on her  all over, her pads of her feet are swollen and crusty.  The vet is suggesting an ultrasound of her liver, chest x-rays.  
How far do we go?? How much more time could we get with aggressive treatment?  What would the quality of her life be???

We also took a punch skin sample of one of her lesions and are waiting for the results.

Thanks.

Answer
That is the toughest decision a dog owner ever faces.  In my profile I caution asking me medical advice.  I have very little to guide you on the possible treatments and outcomes.  There is an Ask the vet category
here that might have better answers.

I am glad you mentioned the quality of life.  You must try to find out what you can.  3 weeks ago we put our 15 year old yellow Lab to sleep.  A few years ago, we spent $1200 having a tumor removed and at 14 she was a frail old dog, but enjoying life.  Last winter was hard on her and she had cancer in her upper jaw.  then this fall, another huge tumor.  She quit eating.  Surgery was out of the question.  The time had come.  

It is tough to lose a dog at 9 years.  Ours was still working as a dog guide at that age.  We had promised her partner that when she had to retire, she had a place in our home as long as she was enjoying life.  15 is rather old for a Lab.  Sooner or later the time comes for all of them.