I bought a 6x10 that had a worn-out, ineffective 5000 btu mounted on the front wall near the floor. It was sort of "squared in" with now-rotten lumber, and then caulked into place. I removed it, destroying the decayed framework in the process, and found that the trailer sides were so weak and thin that there was no way I was going to re-use this hole-- it was a miracle the arrangement ever held together in the first place. I called in an experienced contractor-friend, and he suggested blanking off the hole entirely and installing the new air conditioner high on the barn-style double back doors, which were far more solid. I agreed, and modified the plan by arranging things so that there'd be room for a second 5000 btu if needed, since I live in Florida and conditions can get extreme. (This is far cheaper than buying a 10,000 btu, plus you can only run one or the other if all you need is one, thus saving power. Or so was my theory-- maybe I'm wrong for all I know.)
At any rate my contractor-friend pretty much did the job for me-- I'm not good with tools. He cut the hole, then fabricated a frame all around it out of L-shaped aluminum. We then screwed the air conditioner's own hardware (which will vary by make and model-- this was a Frigidaire) to the aluminum. Then I finger-wedged good quality foam-tape in as tightly as I could into the remaining gaps. The result is a bit ugly but reasonably sturdy, though not something I'd want to hang anything heavy on. Even more importantly, we've been having near-continual, record-breaking rains here recently and I've not been able to detect so much as the tiniest leak, even after driving short distances through downpours.
At first, 5000 btu was definitely not enough in the Florida sunshine at 90 degrees. This surprised me, because the previous owner had assured me that the trailer was fully insulated and that the old 5000 btu had done a wonderful job cooling it while it was still new. Also, when removing the old unit I found a nice, thick layer of styrofoam-like insulation. I soon realized, however, that while the walls were indeed well-insulated the material on the ceiling was very, very thin. When this was removed and replaced with thicker stuff, it was a different story. At 91 degrees in May, in full (intense!) Florida sun-- the most stringent test I've yet been able to perform-- the 5000 btu unit took the interior down to about 78-80 degrees. That's plenty good enough for me, at least for now. So I'll probably never install the second unit. (If you're curious, with my inverter's conversion-load included the Frigidaire pulled about 500 watts at full power.)
I'm not including pictures because frankly I don't know how to post them and think I've done a pretty good job describing my setup. If you feel that photos will significantly assist you with your situation, let me know and I'll try to figure it out.
Best of luck!
