Good evening - I'm Dr. Sara. I'm really sorry to hear that Toby has been ill - I know how frustrating it can be to have a sick pet and not have answers as to what is going on.
In older pets with "mystery" ailments, unfortunately my biggest concern is for cancer. We have tests for a lot of diseases - diabetes, kidney disease, liver disease, etc. However there's no one single "test" for cancer. It can often show up as vague illness with normal blood work or cause nonspecific changes in lab work like an anemia or liver enzyme elevations. Sometimes we will treat these guys just for their symptoms to help them feel better even though we don't have a concrete diagnosis - this sounds like it might be what your vet is doing.
In any case, if you are still looking for a diagnosis, you could move on to more aggressive diagnostics - since you describe labored breathing I would start with a chest X-ray, and if nothing was found there move on to an abdominal ultrasound and potentially some ultrasound guided liver aspirates to sort out the liver issue. These tests can locate masses or tumors in the internal organs that could be causing the issue.
If moving on to further diagnostics isn't an option or something that you want to do, it's OK. An aged pet who is very ill is unlikely to have an easily treatable disease, no matter how many tests we run. It's absolutely alright to consider humane euthanasia here since he is so ill and no longer responding to your vet's treatments. It does sound like you may need to be discussing this soon.
I'm sorry that you're having this difficult time with Toby. Please let me know what other questions I can answer for you :)