I'd guess that you're planning a system that will work when connected to shore power (since you are going to run the A/C to keep the trailer cool). Here is one solution:
http://www.temperaturealert.com/Wire...tCellular.aspx
I'm sure there are a large number of "other solutions" and even some "work around apps" that would call your phone if "that phone" senses a temp rise, and they may work flawlessly, who knows. The system above has been around a long time and is used commercially to monitor "off site" refrigeration and freezer systems. Possibly $250-400 might seem a bit "pricey", but then again, what's the cost we pay for "comfort".....