Greetings!
A direct swamp cooler draws in air, forces it through a water saturated pad, and exhausts it into the interior. These work best in low humidity areas, and work best if the intake is outside air, and there is an open window on the opposite end of your rig.
With an indirect swamp cooler both the intake and the exhaust go outside, but the process chills the water in the reservoir to make it very cold. That water in turn is pumped through a heat exchanger (like a car heater core), and a fan pushes the air through it. In this case, unless it’s getting too cold inside, you keep all the windows and vents shut, and the vehicle will just keep getting colder and colder. It’s like putting your cars air conditioning on recirculate or max. This method does not add any moisture to the indoor air, and works regardless of the humidity levels.
Cheers!