1) You are definitely right about only having one plant per container. You don’t want your tomatoes to have to compete with each other.

2) The leaves near the bottom of your plants were likely turning yellow and dying because of a nutrient deficiency. When tomato plants are healthy the leaves should have a nice dark green color.

3) Try getting some tomato fertilizer that you can dissolve in a gallon of water. Also buy a bag of pulverized limestone. Whenever you fertilize, add equal parts fertilizer and limestone to your water and water your plants with it. The limestone serves two purposes: It balances out the fertilizer by keeping the pH levels from getting too acidic. It also adds calcium to the soil which helps prevent blossom end rot which can ruin your tomatoes.

4) One reason your tomatoes stopped producing could be that the plant was determinate. I am pretty sure that Roma tomatoes are determinate. Determinate tomato plants produce one crop all at once and that’s it. Determinate types tend to be popular for containers because they tend to not grow as large. Indeterminate type plants will keep growing and producing tomatoes until the frost kills it.