J, etc. -
Don't cut yet! One of our members - perhaps harveyrv? - did some nice sheet metal work a couple years ago to address exactly the problem you are having. I'll see if I can find it- or someone else may post a link.
[Edit] I think this is the thread I was thinking of. Try down around post #13 of
http://www.trailmanorowners.com/foru...ead.php?t=7477
The basic idea is that you have to keep the hot air stream being exhausted from the rear (outside) of the unit separate from the recirculated and cooled indoor air flow. If they mix, the result will be no cooling - and maybe some net heating.
In other words, at the front of the unit, room air is drawn into the unit, cooled, and sent back into the room. At the back of the unit, outside air is drawn into the unit, is heated, and is sent back to the outdoors. You must make some baffles to keep these two air streams from mixing. Simple sheets of sheet metal did the trick for Wayne.
Bill