This is obviously very serious, so I hesitate to even throw thoughts out there, but...
It sounds lie a form of stomatitis. Your vet may have diagnosed it as this; however, there are many causes (lupus being just one of them). Other diseases need to be ruled out before this is written off as an incurable autoimmune disease. Most notably, lepto...
Mouth Inflammation and Ulcers (Chronic) in Dogs | petMD
Antibiotics and/or steroids may work for awhile, but with an actual diagnoss treatment can be tailored to try and fix the problem.
I am the last one to go to specialists because I usually find it unnecessary, but there is a time and a place.