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A bit of advice from someone who designs industrial steel buildings:
Consider just using flat sheets, sloped to shed rain away from the doorway of your caravan. You can manually spring steel corrogated sheeting parallel to the ribs, but forget manually rolling across the ribs (like a bullnose awning) - you need a roll-press to do that.
Make sure your roof overhangs the caravan on all sides, and creates shade over the doors and windows, as a lot of heat will come in though these when the sun hits them during the day.
Stand the sheeting off your caravan roof by at least 150 mm, and make sure the air space can ventilate freely, as this will dramatically reduce heat build-up. If it's open on all four sides, you won't need a "whirlybird", the roof will ventilate through natural circulation.
If you're buying new material, go for a light Colorbond, rather than bare zincalume, as it will reflect more incident heat and not get as hot.
Consider putting some fibreglass insulation blanket under the sheeting when you fix it - this will dramatically reduce the amount of heat radiating down from the hot undersurface.
Good luck!
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