So sorry to read about your girl. Enlarged liver can be caused by so many things. If your Vet has done those tests and they were inconclusive, the next step would probably be an ultrasound. I assume he checked kidney and liver function. Did he check for possible Pancreas or bile duct issues? Another, more invasive test, might be a liver biopsy. All these tests would add up financially. Hope you can find an answer.
Please do come back and update.
I can't answer, but I can say we had (decided) to put our 12 year old beautiful and wonderful and loving Golden down - his picture is in my picture file on my profile here. Ours had cancer in fat pockets and removal didn't stop the formation of new cancerous pockets, chemo didn't seem to be an option. We would have, however, kept the dog alive longer is he seemed to be happy and not in pain.
The ultrasound will tell more, we had one on our 11 year old Westie last Decemberr (he is in picture gallery too) - not sure what it cost, it was done at a emergency dog hospital. It showed enough detail to show a tumor had ruptured his intestine - high suspicion of cancer and surgery would have been major, raising a doubt that he could even survive it. For you case the only take-away i think i can give is the ultrasound was expensive, but I think less than $300, and in an older dog it may show only that the condition may require risky surgery and a low probability of improvement. Thus,, my suggestion of giving the dog as much life at home as you can, until pain or other low quality-of-life indicate it more humane to put the dog down I still hurt form those decisions last December, and a few Years before that, and a few years before that.. we have always taken the path of euthanasia based on qualiity-of-life and freedom from pain... much as will be the case for me if it comes to that - i.e., something doesn't just "take me out" quickly.