Fisherman - Good idea on re-using the wiring.
spoon059 - I went outside and measured the opening, and you are quite right, 14x14. Somehow I thought it was bigger. That makes it easy to find a fan or vent cover. Thanks for making me look :D
midnightsadie - While I was there, I turned it on. It is a 70 degree day outside, it puts out 48 degree air inside. Not bad for an A/C that has probably not been serviced since it was new in 1996 (26 years!). Not sure how it would do in hot weather, but it does seem to function.
By the time I sell this trailer, it will probably be worth nothing :D Although, over the last several years, I have had a constant stream of people knocking on my door asking if I would sell it. I had to put a "NOT FOR SALE" sign in the window of the trailer to stop the bother.