How important is it to flush the cooling system with distilled water? To flush my system 3 times would require 30 gallons plus the 5 gallons to achieve a 50% solution.
One possible solution is find a friend with a household reverse osmosis water system. Know anyone with a large saltwater fish tank? The hardcore fish owners will use filtered water in the tanks.
When I did my gas engine I only used distilled for the last flush and the dilution of the coolant concentrate. From what I understand with diesels depends on the engine. Water isn't a high cost item while shop labor is.
I've had my Ford 6.0 flushed along with the Cat C7. A Ford diesesl specialty shop flushed the truck and Freightliner flushed the Cat. They both insisted on using a special flushing chemical and then flushed it once more with distilled water to get the chemical out. The chemical was to remove any rust particles which flushing with just distilled water won't do.