Not clear what problem you're trying to solve.
If you're in the trailer and want to know who's at the door, most people would look out the window adjacent to the door. But maybe you don't have a window that looks out on the entry.
If you're away from the trailer and want to know who's at the door, you could use a Ring doorbell. As mentioned, reliable WiFi is a must for remote usage. Also, you'll either have to run 12V to the Ring doorbell, or get a model with internal battery and change the battery periodically.
If you're leaving the trailer in a setting with nothing close by, you might enable motion detection. But if people or wildlife are near the trailer, you'd be likely to get a lot of motion detection alerts.