I just did mine this winter. A complete flush of engine, hot water and bus heater. I purchased 2 gallons of coolant. I did a true flush of the Hot water and bus heater and filled that system separately using the shut off valves and a pump then filled the engine coolant system. I did this during the winter and I assume most of the air has bleed out of the system simply by time. I now need to add about 1/2 quart of coolant. I would purchase 3 gallons to be on the safe side. It's good to have extra coolant on board to top off or because of a small leak.
