I mounted mine without cutting the side of the trailer. Many others have as well and done it better than me.
Basically the area where cool out side air comes into the AC needs to be seperated from where the hot air exits.
The cool air comes in on the sides and top and blows thru the rear of the AC unit and comes out hot.
I have a hole thru the floor below where the hot air comes out. I use two small muffin fans to blow the hot air out.
I seperate the cool section from the hot section with a piece of foam that fits between the closet walls and air conditioner. The cool air comes up thru a seperate hole in the floor. Again it needs to be seperated from the hot air under the trailer.
As Jim suggests the AC needs to be tipped a little towards the rear so it drains towards the back. I 'gooped' a hose into the drain hole of the AC to drain the AC unit thru the floor.
If you look at Preparing a 1991 Scamp
you can see my pictures. I'd be glad to provide more pictures though after I did it someone else did it more professionally than me and had some great pictures. Unfortunately I did not bookmark his thread.