[CarKath]

If you can, test the 30 amp to 15 amp dogbone. Or get a buddy to do it if you don't have a multi meter. Lot's of how to use multi meters out there.

If that tests out ok, keep tracing it back. Umbilical , shore power plug on trailer, pull it out and clean / tighten connections.

Next would be to trace that to trailer panel to make sure it is not shorting out along the way. Maybe a rodent got in and chewed it, never know.

Basically you need to inspect from one end to the other and test it along the way.

My best guess is the 30 to 15 dogbone is NFG.