Swamp coolers work great in the desert. In areas with high humidity, they just leave it hot and damp. Guess which one Michigan falls under in the summer.
You should be able to fit an aircon unit on. They aren't custom to a particular model.
We used a portable aircon on out boat for a while. The problem with a truck camper is it eats up floor space which most truck campers start out with very little. Also, you need a window where you can vent the exhaust tube. If you get one, look for the twin tube versions. The single tube uses already cooled interior air to cool the condensor, so they aren't nearly as efficient.