Evaporative cooling only works in low relative humidity. Completely useless in the eastern 2/3 of the continent. Even in AZ it’s not much use during the muggy monsoon season (July-August). I had it in a 27’ trailer I lived in full-time for four years. It increases indoor humidity, and no matter how often you clean the thing and change the pads, it always seems to have a funky damp smell.
Even under the right conditions, I’d be surprised if a small 12V unit could move enough air and water to cool even a small trailer.