In several of my previous RV's I have found that in very hot weather, the more ventilation I could get around the back of the AC or refg. the better the cooling.
Over on the Canned Ham Trailer web site, there is an excellance 3-part video where Larry installs a AC in the side of a 1958 Deville (non-fiberglass) trailer.
There is a lot of good information within this article, and while a great deal of it might not apply to a fiberglass trailer...knowledge is knowledge!
However, some of the methods that are shown are not really necessary.