It may be a very simple fix. Remove the screws to the AC grill on the interior of the trailer. Look up inside and see if a panel has moved. What happens is that if the panel moves, the cold air immediately circulates back up the intake side, and the AC sensor thinks its down to the set temperature. If it's moved, reposition and tape in place with aluminized duct tape.
Of course, this advice could be incorrect depending on the model of AC you have. But even so, I'd check for a path for the cold air to be immediately returning to the intake. BTW, your dust filter is clean isn't it?