Hello there
If it is not cancer then it could be an infectous cause.
Have they tested her for all the tick diseases? Erlichiosis? Lymes? Rocky Mountain?
Have they tested her for toxoplasmosis? Blood test?
Did they do a bone marrow biopsy yet?
Is she on any antibiotics or prednisone?
It sounds really bad. I am sorry to say.
Is she on doxycycline? That would kill a tick disease. Maybe the vet is treating for it anyway.
I was thinking acute lymphoblastic leukemia.
More common in females. Average age 6 years.
Signs: large liver and spleen, enlarged lymph nodes, other organs may be affected.
High liver enzymes.
Low red cells, white cells, platelets.
I will relist your question.
Veterinarian
Veterinarian in practice for 10 years. Compassionate, Caring, Experienced.
I think she needs a bone marrow biopsy. That disease I just told you about to diagnose it you need a bone marrow biopsy.
In my experience, in the older pets, the sickness comes on so fast there is nothing you can do. Often it happens over a weekend. I can tell you love her very much or you would not have been to all the specialists trying to find the answers. I do not think you missed anything in her behavior. It is probably her time. 12 is the life expectancy for her even if she was not sick. The best thing you can do for her now is keep her comfortable and then when the time comes she is not comfortable, let her go.
That will be your gift to her.