Nonregenerative anemia this low is certainly indicative of a severe problem. Nonregenerative anemia can be caused by many different problems. They include:
Chronic kidney failure
Chronic liver disease
Immune-mediated destruction of red blood cell precursors
Iron deficiency (which is usually caused by chronic blood loss)
Bone marrow infiltrative diseases--myelodysplasia, neoplasia (cancer), myelofibrosis, etc..
If your dog had chronic kidney failure or liver disease, this really should have been evident on the general blood and urine tests that the vet likely already ran (CBC/Chemistry Panel/Urinalysis). If your dog is having chronic bleeding
, this is usually into the gastrointestinal tract, and thus there are often other signs of this like dark tarry stool and sometimes vomiting and diarrhea. Also, there would be other signs of this on the bloodwork that would have suggested an iron deficiency problem. You mentioned that your vet ruled out tick-borne illnesses. I am assuming that means tick titers were run, so ehrlichiosis would have been ruled out. This would not rule out babesia infection (although this is not very common in the U.S.).
If a bone marrow biopsy is done, this could tell you exactly what is causing this problem (whether it is immune-mediated, cancer, or some type of myeloproliferative disease). If this is an immune-mediated disease, it may just be that the pred isn't enough. Often, dogs with this type of problem need to aslo be on a drug called Azothiaprine. You may want to discuss this with your vet.
If it is not immune-mediated, the bone marrow biopsy will not likely diagnose something that is easily treated. Some cancers do respond to treatment and others do not. If there were pancytopenia present (other blood cells are also low—like white cells or platelets), then there is an extremely guarded prognosis.
I hope that this information is of help to you, and I wish you the best of luck with your dog. Please let me know if I can be of further assistance.