The trailer wiring may be your problem. Check where the wiring goes inside the trailer axle. The rubber grommet may be gone where the wire enters the axle tube and is shorting. There will be a hole near each axle hub. 4 holes total.
The wire inside the axle tube sometimes wears and shorts. Requires replacing the wire and I suggest routing outside of the axle.
If you have the fault most of the time disconnect one axle at a time and see if the fault clears.
When making the connection either use a butt connector and make sure the wire is corrosion free or solder the connection. Then use some marine grade heat shrink to cover the connection. Harbor Fright has the marine heat shrink or any marine supply house should have it.