Totally understand the dilemma. It may take quite awhile to not only locate source of pain but for pain meds to build up and act in conjunction with each other. And no one wants to see the one they love in pain. So...

Specialist may be necessary at this point: whether pain management vet, rehab vet, or teaching hospital. 

Also, I know it sounds strange and unusual but one of mine is on the highest end doses of a cocktail of pain and antiinflammatory drugs and has been since Thanksgiving ( he is not a TriPawd but does have a compromised leg). So he takes Gaba, Tramadol, Previcox, Amantadine, and a stomach protector like Nexium. It isn't perfect for him every day but it does work the majority of the time. It took awhile ( a few months) to come up with the correct combination. 

However, if there is something else going on, a specialist should be able to determine that and then you will have an important part of the puzzle. So it does sound as if this is an instance that a specialist may help. Also, the Quality of Life scale ,for many, is a helpful resource. Hang in there and hang tough. We are sorry to hear of how hard it has been and hope that you will find answers soon and that they are only good ones. Sending best wishes, huge hugs, and PyrPaws all around!