I'm not sure how big a trailer or just what your needs are, but they make self-contained units that sit on the ground and connect to the trailer with duct work.  Most are heat pumps that heat and cool both.    It may be too much, or may be over budget, but it's an idea to ponder on.  Here is an example I found with a quick search.
http://www.alpinehomeair.com/viewproduct.cfm?productID=453062012