I had the same issue with mine; found it difficult to get the water pump to draw through through the winterizing port. Make sure you have a good seal at the hose attached to the winterizing port, keep the intake hose as short as possible and elevate the container of anti-freeze. Finally worked for me.
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Scott & Alta
2017 Montana 3160RL, Legacy, Onan 5.5Kw, Solar
2022 F-450 Lariat Ultimate
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