Need help from others with window/wall style ac units.

I have a thru the wall ac on my 17 ft tt and haven't had any problems in three years. If you were to mount this ac in a window in your house it would be surrounded with a metal frame except for the back, so I'm not totally convinced that's the problem. Did you have the same problem with the original ac? I know it didn't cool enough, but anything other than that ?