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Evaporative coolers, sometimes called swamp coolers, are an option in areas with low humidity. They do not cool by a certain number of degrees, but rather output air ~5-7F above wet bulb thermometer reading. Standard sizing is cooler cfm = cubic feet of area / 2.1)
Water consumption for a van-sized space is about 1 quart/hour of cooling.2)
The main manufacturer of RV swamp coolers is Turbokool.