The damage is hard to diagnose with certainty, but it resembles Septoria Leaf Spot, and appears to be significant, even for a fairly common tomato disease. There may also be overlapping Early Blight, another quite common fungal infection for this crop. Some cultivars might have better resistance than others to wilt or other diseases, though your practice of removing lower leaves as the plants matured was helpful to reduce vulnerability.

We don't see an example of any wilt diseases specifically in the photos, so if plants are wilting, it might be from drought stress rather than infection. It sounds like each plant might be receiving only about 1 gallon of water per day, split into two sessions, which might not be leaving it with enough moisture in the morning to stay hydrated throughout the day, especially with this year's heat wave and insufficient rain. You can feel the soil around four inches deep next to the roots to judge how far the irrigation water seeps in, and if each watering session is rehydrating the soil to that depth. (If not, consider increasing the soaking time for the morning and discontinuing the evening watering.) The vegetation (weeds?) growing around their base may also be robbing them plants of needed moisture, and mulch may be a better groundcover alternative between plants for future seasons in order to help hold moisture in the soil for use by the tomato roots.

Is this a home garden or a small farm? If a commercial operation (or a non-profit...anything other than a residential garden for home use), we recommend you get in touch with your county Extension office's agriculture educator, Dave Myers, as he may have more resources for assistance.

Miri