I would like to see the results of adding a proper veg fert and not using ro water. Seems like there is little left to try if not heat, i would like to check the pH too as these variables make a lot of difference to the grow. Outside gardeners lime the gardens every two years in the winter before the summer they wish to grow in. Lime can take a while to work outside and ro water will rob the plant and soil of the salts especially cal/mag. Why not work these problems and see if you get results?? WOuld be what i would do in that situation.
Is the problem getting any worse? Any recent pics please?