I have one in my PLAN B box...

This is a schematic of what they look like... You can see where only one leg of the 50AMP service is tapped by the 30A female socket.
You will be plugging into a 50AMP source but your trailer is protected by its 30AMP main breaker.
If you try to use more than 30AMPS it just trips your trailer breaker.
We also like to have a good 20AMP Extension cord along with us as well. We will plug into the camp ground 20AMP SERVICE to run all of our outside items and portable electric heater when it starts cooling down at nights.
Roy Ken