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Can you post a picture of your plug? I have never seen an Airstream with a 240VAC set-up. Plus, the only RV's that use a 240VAC plug are the bigger MoHos and 5th wheels that use a 4 prong 50 amp plug.
What you should have is a 3 prong 30 amp 120VAC plug that is RV-specific.
A picture is worth a thousand words though. You might check it by seeing how the umbilical's wires are connected into your AC breaker box. If the black lead goes to the hot bus (breakers), white goes to the neutral bus and the green or bare wire to the ground bus, then you have a 120AC set-up.
Keep us posted.................