Do you have more then one gfci in the trailer?
Many times they will run them in series and when the first one clicks off, the second or third in line also don't work.
However, Most of the time when that happens, there's only one gfci in the circuit and the outlets following are gfci'd are also protected.
Also as you indicate that the breaker wasn't tripped, I hope you meant the one on the gfci, not the panel breaker?
Bob and Better Half
2007 F-250 PSD, SW, CC, LB, 2WD
2006 TF32SS Cross Terrain Toy Hauler (sold)
Reese 16k slider