NOT sure why your BrakeSmart is going in to an error mode, but simply judging from the fact that it does so with 2 specific trailers, and NOT with 2 others, then I would have to venture that it IS the trailer wiring, not the Brakesmart, despite your efforts at rewiring the trailer.
I DO like BGSmiths idea of running a separate ground wire for those trailers that are exhibiting the problem, whether or not they are tilt trailers. ALSO, if for some reason you used ScothLoks connectors on the trailer wiring, I would go back and use solder and marine shrink tube on the connections. I owuld do all of that before buying another controller.
On the subject of PlugItRigjht no longer being a MaxBrake distributor, it look like that JUIST happened 3 days ago... they DID leave a direct contact phone number for MaxBarke on the same webpage... .
The phone number for Max Brake is 337-328-2264 e-mail:info@maxbrake.com