I reckon they've just got to the end of their life for this year, mine have done the same, I just pull the leaves off, cut off spent branches, and stop watering to get the last of the fruits to ripen.
I've probably got several kilos still on the plant turning red at a slower rate, but am still picking sufficient fresh toms and am chuffed for the time of the year.
I never did any PH testing, as I just assumed they would die off, as yours have done.