I have not done this yet, but on my bus the RE means there's a huge radiator screen, the back emergency window, another window on top the emergency window, all the safety lights and I added a tranny cooler on one side of the radiator.
I have no space along the sides due to my storage units.
But, I have "just" enough space to the right of the radiator screen to mount it, just above the bumper. I'd prefer it higher, to avoid as much dust and such from getting in the outside unit.
The evaporator will go on the inside, at the back, just below the ceiling.
I've seen many who have space along the side where they can cut an opening in their skirts.
I think this might be a good idea since you can more centrally place the evaporator inside.
Sometimes it's less about the best place to put it versus where you can put it.
Look forward to see what others have done.