True story: Man came to me "Half my electrical outlets don't work" I ask if he had an inverter (he did not know but we found it,, This was on a class A) The breakers looked somehting like this \/ Not \\ or // but \/. When he went back inside his wife ask "Does he think he can help" and the husband answered "he's already fixed it" (Turned on the off breaker).
Now to your problem.. Generally Trailers DO NOT have Inverters factory installed. (nor does MY Rv but I have one) But they do have "uni-box" or "quick-box" outlets.
With this type of outlet instead of strippign the wire you just strip off the outer jacket and then jam the wire down in a slot. the metal edges of the slot cut through the insulation and make contact. Works great for a 100mA or less phoen or Teletype line but for a 10Amp plus (power circuit.... not so good.
Then they shove a 2nd wire on top of the first and that runs off to the next outlet
====||===||====|| style (|| is an outlet and === is the wire)
The connections can melt.
Start with your test device (Light or meter) at the GFCI. pull it usually it has wires around a screw or poked into a hole or wire-nutted and if it's wire nutted that's suspect #1 Make sure you have power in and out.. then map out what works and what does not. Pull the boxes. open them and check for power using your test device.. if none at all. Close that box and `move on to the next. Best I can advise.
If you have a slide-out with an outlet there will be two junction boxes under the slide.
NOTE If you are NOT comfortable working on live circuits (I am and tend to forget to say this) Get professional help.. Live 120 volt circuits can be deadly