Need some clarification

I am stage 4 metastatic melanoma being treated with Nivo. I have had 9 treatments. I have been very fortunate to have experienced very few side effects: varying degrees of exhaustion, occasional rash, some gut pain, and that’s about it. I have read other patient entries and I am fully aware how luckyI have been in my treatment. I had an original mole on my upper back that was excised five years ago and I thought it was resolved. About two years ago a tumor was discovered in my back and a biopsy confirmed subcutaneous metastatic melanoma and it was excised. About six months later a small amount of bone cancer was discovered in my right scapula and this was diagnosed as a further metastasis. It responded well to radiation. During my 9 months of Nivo there has been no further metastases— no tumors, no obvious spread of cancer. Here is my question: Because my melanoma is subcutaneous with cancer cells flowing through my lymph system or fatty tissue undetected, am I at risk of dying within the time estimates on the longevity charts even though there are no positive cat scan or pet scan results of new tumors or organ involvement. I do not feel well and can’t point to any new event that explains my rundown condition. I am a young 75 year old experiencing a lifestyle change that is very frustrating. It clearly is not age that is causing this. When cancer takes a victim, can it include someone who has no persistent tumor growth or organ deterioration? The doctors tell me my scans and bloodwork are ok My body says otherwise. What do you think?
(I know many of you have experienced significant medical challenges with your melanoma treatment and may view my concerns as trivial. I do respect your courage. I am trying to formulate some end of life plans, but I simply cannot understand my true status given the subcutaneous nature of my melanoma..) I look forward to your views.