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Since the 50 amp service has two sides feeding the circuit breakers in the trailer and you cleared the problem by shutting off the outside breaker and re-seating the plug I think you will find one of the pins on the out side plug may have been arching or one side of the breaker did not have a good connection.
The fact you had a working condition by moving the GFI breaker in the trailer points to that suggestion.
This is my assumption from what little I know about electricity.
Good luck and glad you got it working.
Cheers,
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