Are you certain you found all the GFCI's? Not questioning your knowledge of your own trailer, but something similar happened to me a few years ago. It was a new-to-us TT and the manufacturer had placed another GFCI in a bathroom cabinet. It was hidden and turned out to be the one that I needed to reset (at the time).
If its not the GFCI's, or the breakers, or fuses, than that leaves you with either an internal wiring problem such as loose connector at each receptacle and the light or at the converter or a short somewhere between the converter and that part of the trailer.
DW, 3 Kids and our Goldens
2012 Shamrock 233S
2008 Toyota Sequoia 5.7L 4WD