Welcome to the Forums!
Do a google search from outside of the AirForums and look for search terms like "refrigerator troubleshooting." I know there are some detailed threads here describing the steps to go through to troubleshoot RV refrigerators.
The problem could be simple, ie., the electrical heating element not working, and a cheap and easy fix ($50). If the coil has allowed the ammonia gas to leak out, then the fridge isn't worth fixing, and its only value may be in a few of the plastic parts.
If you have pulled the refer out of the trailer, put it on a nice level surface, plug it in and turn it on in electric mode for max cooling. You will have to wait around 24 hours for it to feel very cold inside. The electric part of the cooling mechanism is just a heating element that is installed in part of the piping in the back. If you can feel the piping getting hot after a couple hours, but the fridge is not getting cool in 24, then you have probably lost the gas from the system, and you need to consider seriously if it is even worth fixing (replacing the cooling core will cost about half of the price of a new fridge). If the piping in the back is not getting warm, then either the heating element is shot, or wiring, thermostat, etc., is. At this point, try attaching it to a propane bottle and lighting the fire and running it in propane mode. If it gets cool, then great, it might just be a minor electrical problem, if not, then, again, you've lost your coolant.
If you decide to repair it, have a good look at the rubber seal around the door and the condition of the interior. If the seal is shot, and you have cracks in the interior and missing shelves, then you still may be looking at a lot of expense to getting it working well.