can you post some pictures?

You say "large" plastic containers. How "large" specifically and what variety? The typical cherry tomato plant gets huge and to thrive it would need a container the size of a half whiskey barrel. Your plants may just be root bound and so dying off since they have out-grown their containers.

Dave

You can measure your pot in cubic inches, then divide by 231. That wlll give you the size in gallons. [231 is the number of cubic inches in one gallon.]

Another way to calculate it is that one cubic foot is 7.48 gallons.