Yeah I am aware of making a living getting in the way of doing things, well past that now. Sleeping not so much but eating, usually on the run, for sure.
I remember you saying the brakes on your other trailer were still working, so the truck is out of the picture. I agree the most likely problem with the break-away in the electronics of the battery box.
At least you have crimp connectors, all the connections on my trailer when it was bought were vampire splices like these.
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IMHO they should outlaw those things, the way I see it manufacturers use them for cheap fast connections that will work long enough to get the trailers off the lot.