Here's something else I thought of. I remember a post where the GFCI was not tripped, but the load wire (wire going down to additional outlets) had burned. You might want to unplug the trailer and pull the GFCI outlet out to take a look.
If you have one of the $10 plug in testers with a GFCI test button,
link Try plugging the tester into the working outlets and press the test button. If the GFCI trips then it's not the problem. But if the GFI does not trip on any outlets then I'd say the problem is with the GFCI or wiring coming out of it.