A good score on that window AC. I think that if you can fit it up there, it would be a good location.
A member (diesel Dan) made some ductwork for his AC. He made it out of wood. The duct could get the cold air to the middle or the front of the bus.
If I were to make a duct, I would glue Formica to the insides.
The problem is that you lose headroom.
I would mount an AC in that place, however, there is no room for one in that place on my bus.
Your best bet would be to use high velocity fans to move the air around.
I have an AC that size. It works great. I mount it in one of my side windows. But, I do not drive my bus much. I take it out for the winter, and of course, for driving.
I have a high velocity fan that does well. In the winter, I use it to circulate the heat. In the summer, I use it to circulate the AC.
This fan really puts out a powerful wind. It also oscillates if you want. I have been using it for over four years. You can feel the fan from the front of the bus to the rear. This particular one may be the srongest one that Lasko makes. They are still available, and they are around $50.
It is model 4924.
Go to the Lasko site for prices.