Best Way To Add A/C to a older trailer?

I agree that a Window AC unit is the cheapest and possibly the best option. But,, unfortunately,, these days with the way people are you will have to keep it hidden at a lot of parks. People will "Look down their nose at you" if you have a window unit sticking out the side of your camper. And, some parks will not allow it.

If you build it into the camper as Gdetrailer has said, and you keep it looking nice then that is helpful and you should be OK.

Have a nice Day