Sick Dog with mysterious illness

Question
Hi there. Just wondering if you could maybe give me some advice or help on this subject. I have a 10 year old lab/chihuahua mix that is ill. About 2 weeks  ago after we moved to Az from Nv we notice him starting to shake a bit. Like in the legs only. I thought it was nerves. But then he started to urine more frequent and drink A LOT. Plus went from 35 pounds to 20 pounds in the last month. Then last week it got worse and he collapsed in my kitchen. Fell right on his side onto the tile and then urinated. I took him right to the closest Vet, who ran tests overnight (and an X-ray). He found a high white blood cell count & low red blood cell count. But all of the other tests came back fine. I have a list of the tests he performed if it helps. They gave me a steriod in pill form (every 8 hours) and some liquid medicine to help his tummy (every 6 hours). This was Friday. Since Friday his stool is black & tar like, he can't stand more then 20 seconds before falling over, he is constantly moaning/crying in pain (of some sort), can't eat on his own (we spoon feed) & we have to physically carry him to the yard and told him up to urinate. I don't know what to do. We have spent $779 on tests last week and I think he may be dying. We did not get any diagnosis or anything, so I somewhat feel cheated. If there is any light you can shed on this, thank you. I am worried and deeply upset at my dog "Marley's" state right now. In 2 weeks he has went down hill. I feel as though I may need to put him to sleep. he can't live like this for much longer. It is so sad to see it. Thanks, Shawna

Answer
That weight loss is dramatic and I would be concerned about cancer.  The black stools means that he is bleeding in his stomach, and explains the low red blood cell count.  With the low red blood cell counts, he won't have any energy.  My guess is the vet put him on steroids for possible autoimmune hemolytic anemia, suspecting that his condition would improve with the medication.  He sounds like he is in a very poor state and euthanasia may be your only option unless you take him to a specialist.