[ChuckS]

Considering the age of your fifth wheel (2015) when is the last time you:
1. Remove each hub and performed maintenance and checked the condition of the brake magnets and wires attached to each magnet?
2. I would also disconnect the wires on each magnet and ohm out to verify
3. Condition of the wires inside the junction box for the Pin box for good connection to the seven way cable to the truck connection

I would just have to know the overall condition from the ends of the axle first and then address the wiring to the four brakes... All are wired in parallel

And check the ground connection that each of those brake magnets are tied to for corrosion, etc