in your post you mention all the outlets in the trailer, are all those GFI, usually GFI are within 6 feet of any water source, which in a trailer means all outlets are about 6 feet from water, now the questions is are all the outlets GFI's? if they are then to test if they are all connected when one trips all of them should trip too, if that is the case then you need to isolate the bad one by reseting all the GFI outlets and then plug a light and turn it on, one outlet at a time and the bad one will trip when the light is plug and turned on, once you find the bad one then all you need to do is change that one only, however if there are more to be tested then reset the GFI again and continue testing the one that needs it, if another is bad then you know you need to change more than one.