I'd go for a 6000 model Fantastic Fan. It not just has a thermostat, but a rain sensor. It also has a variable speed control, and can be set to move air in either direction. There is a 7000 model, but it isn't 12 volt, but requires a full 120 VAC to function.
Some vans have screens. I know that SMB has it as an option, as well as Roadtrek. That way, the rear doors or side sliding door can be left open for air, but still keep bugs at bay.