Subwoofer RCA Connectors
You do need one of the RCA connectors. I would get a RCA Y connector for the radio side.
One of the plug wires is an actuator wire. It turns it on. On my last trailer the wire was cut from the factory. The plug has power and a trip to turn it on when the radios on.
My trailer was wired wrong at the TV as well. You should be able to play surround sound for it, via the radio. The RCAs go via a Y into the sound OUT jack. Mine was wired to sound in jacks.
If you could have a superpower, what would it be?
I chose America