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.