About Seven years ago we built a mobile hunting camper out of a new cargo trailer and installed an air conditioner similar to this:
http://www.homedepot.com/h_d1/N-5yc1...atalogId=10053
The ac sort of looks like a rolling piece of luggage with a hard plastic case.
We built a triangular 2x4 frame in one corner of the cargo trailer and also used 2 long bungee cord to help hold it in place. It never moved. Tonight after I get home from work I will email you a photo if interested. Ours frame was not fancy but perhaps it will give you an idea of something to do.