Could be the fan motor or a number of other things. If the unit throws a breaker, it's pulling more power than it should. If you're handy with a multi-meter, then you can do some electrical testing. If not, then you can do some rudimentary troubleshooting using your eyes and ears.
For electrical testing, check your amp draw on the power lead going to the AC. You should be able to get a loop multi-meter around the wire in your breaker panel to test for amp draw. If you have a 13.5K BTU, you should only pull 14 amps max during run. If you have a 15K BTU, you should only pull 16 amps max during run. Startup amp draw will be much higher, but only for a half-second or so. Any more continuous amp draw than the norm is an indicator something else is wrong, which will eventually cause the breaker to over-heat and trip.
Also, check your incoming voltage. Low voltage will do funny things, so make sure you have AT LEAST 110 volts coming in while the AC is running. Typical voltage should be around 120 volts. If the voltage is low, your amp draw will be higher which would cause the breaker to overheat and trip.
Last---or really first----make sure you're hooked up to a good power source using appropriately sized cables. If the AC works fine in one location and trips at other locations, then it's likely a power issue.
If you don't have a multi-meter or aren't handy with electrical stuff, you can still observe a few things to get an idea of what's going on.
Try pulling the front of breaker panel and running the AC. Once the breaker trips, feel the breaker to see if it's hot. If it's hot...like so hot it almost burns you, that's an indicator of either low voltage, low amps, or much higher amp draw than normal due to a malfunction with the AC unit.
Inside the camper, pull the AC cover and check your filters. If they are extremely dirty, they may inhibit airflow which would cause the motor to work harder to pull air, thus causing more amp draw and overheating the motor and tripping the breaker. While your filters are out, shine a flashlight on the coils to see if they have an ice buildup or if they are dirty. If there's ice, it could be caused by poor airflow or it could be caused by low refrigerant. Poor airflow is typically caused by dirty coils or dirty filters. Low refrigerant means it's time to replace the unit.
If able, you may want to get up on the roof and pull the shroud. Run the AC, and then observe it as it slows. Kill power, and then see if you can spin the blades with your hands. Note---you have to kill the power first or you'll wind up being known as Lefty...or Righty...depending on which hand you used to spin the blades.
If the blades spin, then the motor isn't seized up. If the blades spin but make a noise, even a slight rubbing noise, then the motor bearings may be shot and you'll need to replace the motor.
Check the outside coils as well (rear of the unit). If the coils are extremely dirty or coated in ice, it could cause poor airflow. The motor would draw more amps and cause the motor to overheat and the breaker to trip. If dirty, clean and try again. If coated in ice, that's a potential sign of low refrigerant which means the unit would need to be replaced. It's also a sign of low airflow.
Bottom line, the motor spins for 30 minutes, so it's probably not bad. It may be weak...but not bad. My bet is on the coils being dirty which causes poor airflow, which leads to coil icing, which leads to more amp draw, which leads to the fan overheating as it draws more amps, which leads to the breaker tripping.
Not that I'm not an expert on this stuff. If you're uncomfortable doing troubleshooting, then I highly recommend taking it to a professional