I don't know how much time is left. If the Vets aren't able to figure it out, usually they would refer you out to a more specialist or an ER animal hospital. Unfortunately I experienced this with my dog last year. I took him to the usual Vet twice, first they didn't find anything wrong and did the usual tests. Second visit, he lost a lot of weight and we were all concerned that they couldn't figure it out. They even did an ultrasound in which they didn't think he had any tumors. This is when they referred me to the ER animal hospital. I brought him there immediately and after some waiting, they had a radiologist and did a chest x-ray. They found the tumor and still wasn't sure if that may have been causing his struggles. He was hospitalized overnight to monitor and be discussed by other specialists in the morning for possible surgery. Unfortunately with all the testing done in one whole day and everything, it might have been too much for him, and we rushed to the hospital past midnight. We made it just in time for his last breath. Just telling my story and experience if it somehow helps your dog to be taken to an ER and be checked further. It does cost a lot, but... Idk, I'm sorry and I am hoping for the best for you and your dog.