Take it to the Airstream or other RV dealer, or a shop that sells and services large trailers like horse trailers. Electric drum brakes look similar to hydraulic drum brakes but work differently and could confuse a less than sharp auto mechanic. A sharp one could probably figure them out, but then might not know how to ohm them correctly or have replacement parts.
A new pair of magnets shouldn't be that expensive (or used to not be) and the total labor cost shouldn't be that much more than for R&Ring the wheels and drums and troubleshooting.
I forgot to mention it, but I'd take a look at the wheel bearing grease while in there too. (honestly, if I was that far into it, I'd repack them)