Most of my tomato plants are doing really well. But the one in the cage highlighted in red started failing a few weeks ago. I fertilized with some Miracle grow I had lying around and it didn't improve. Then I got some Tomato Tone. That did nothing. The plant kept getting worse and worse and now it's just a husk. I'm afraid blue is following next and I'm concerned about purple. Here's another angle for some more context. 
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Any thoughts?
Nah usually it's my fault.
Since you tried nutrition I would suspect injury to the plant has allowed some kind of infection in. I would try trimming away all of the diseased or damaged plant matter and then applying a copper fungicide. I've had tomatoes with vines split fully in two and the plant stayed productive and vibrant once it got far enough from the damage and new growth was happening.