Some trailers have their thermostats mounted where they are affected by the conditioned air. So, at the thermostat, it is cool enough for the thermostat to signal the A/c unit to turn off. But I'm thinking your problem likely is different. Are you using the high fan speed? I know there isn't a big different between the two. A common problem is between the intake side and the output side of the ac grille. There is a divider that should completely separate the return air from the conditioned air. The divider often slips and the conditioned air is sucked back into the return air, making the a/c ineffective. When the unit is running, is it as cold as you expect it to be? Sorry for the questions.