Hate to state the obvious, but test the pH of your RO water. Test the pH of your grannies rain water and compare. This will likely give you your starting issue. As has been stated by many already on this topic, your bitterness may have to do with astringency because of pH at mash, rather than the pH at boil.
By the way concrete takes a while to be eaten by rain water (and why would you be puting RO water in concrete I don't know). Concrete rain water tanks, and many other factors in rain water systems tend to neutralise pH in rain water from 5.6 up closer to 7. Falling from the sky, 5.6 maybe, landing on galv or tiled roofs then into the dirty gutters then into the dirty tanks = increase in pH.
EDIT - you state your pH meter is broken, but then talk about using your pH test strips. USE the test strips to test your water. Better than nothing.