A three wire detector is for connecting to "house" and "chassis" battery banks. That way if one source dies the other can keep it going. I believe Fleetwood used these and it should have a diode in the circuit to prevent draining of chassis battery bank over a very long period of time.
Mike, RVIA & RVSA Certified Master RV Technician
Amy, Dr. Assistant - Roxie & Mei Ling, four legs each
2000 Gulf Stream Scenic Cruiser 450 hp & 1330# torque
06 Saturn Vue, 06 Chevy Z71 4x4 & 2014 Corvette Z51 M7