You may have tried/checked already already but just in case:
Make sure your trailer is level, if it's not the fridge won't chill as well.
Make sure the evaporator fins inside the fridge aren't iced up.
Make sure the exhaust is clear
Make sure its not packed too full so that the air can't circulate.
If the burner is dirty or not operating properly it will cause problems, probably need a qualified service person to check that though.
If none of these help you can buy a small battery operated fan to put inside for a few bucks that may help somewhat, and you can also install a fan on the exhaust to move more air over the coils that may help.
I'm sure there's other stuff that could cause the problem, hopefully someone with more experience will chime in.
2011 Chevy Silverado Crew Cab 4X4
2009 210 WBS