Welcome to iRV2.
First post I see.
First thing to do would be to get underneath ,to the backing plate for the trailer brakes and check the wiring going to the magnets there. If there is any sign of melting of the wiring there.
Personally, I'd pull all the wheels and brake drums to test magnet operation, and examine for heat damage; cracking .
Probably time for a wheel bearing pack anyway.
The magnets can be tested with the drums off by powering them up and seeing if they hold a wrench.