If the summer is above the 105 or so mark there is not much and I've had the same thing, but keeping the root area cool seems to help the most.. I've never had great success with container tomatoes or raised beds because of the heat. Very heavy mulch that breaths, like wheat straw, Christmas tree mulch and one year my son in Las Cruces brought me a load of pecan hulls. I also use overhead sprinklers at 1pm for 30minutes each day when above 100deg. I think the evaporation does more for cooling than anything else. Also the city water is so hard that there is a noticeable difference with our well water or after a rain shower ... the tomatoes just do better. I've also had better luck with plants that are heavy leaved and provides some shade for themselves. I have a friend in town that has built a shade cloth awning over his whole garden, but I gave up on that a long time ago because of the monsoon winds ... and I really do believe that the cool roots are the biggest advantage. The tomato plants themselves handle the extreme heat well enough.
You mentioned BER.. It may be because of the containers and stress, but you might try only in one container for an experiment a half solution of the stuff called Cactus Juice.. Cactus plants need a lot of calcium and most cactus fertilizers have it available.. it is a liquid sold at HD and Ace and Lowes.. I also use this stuff with liquid kelp again at half strength during the heat as a spray at daylight once every few weeks. This gets involved and only experiment with one to see if it will work for you.
