Drop the notion of pre existing wiring convience. The critical path is from the solar controller to the battery where small losses can cause significant loss of battery charging. Mount the controller as close to the batteries as possible.
12V panels produce about 18V and that wiring can tolerate significant voltage drop w/o power (battery charging) loss if you're using a PWM type controller. MPPT controller are a different consideration.
The key paths are panels to controller and controller to batteries. The refer and it's DC wiring is not involved.