Tomatoes (and potatoes... same family) are affected by any number of fungal and viral infections. It's hard to say with certain which is the case here, but the kind of discoloration, progressive loss of leaves and so on is not uncommon. Here is factsheet to help identify tomato problems; there's a lot more issues described here than you need to look for, but the images and descriptions (and how to deal with them) are good: http://extension.colostate.edu/topic-areas/yard-garden/recognizing-tomato-problems-2-949/