Hello: A general question...
Old old Magneteks. Not as old as me but nevertheless impressively obsolete...
Did not these units have a heavy wire or wires to power the fuse panel thereby power the camper loads, lights, water pump? This circuit could never see a direct path to the battery.
A separate circuit, one wire feed the battery. This circuit came from a huge ceramic bleed resistor inside the converter and the most I have ever seen for a charging rate was about three for four amps. I named it "The resistor bleed circuit".
Thus the two separate output wires could not exchange duty. The load wires could not connect to the battery and verse visa. Extrapolating, the OP's two wires are to feed a load fuse panel, while the remaining red wire is connected to the battery? White of course going to chassis negative?
EXTREME care must be taken with these old systems because as the ferroresonant voltage control capacitors age, finishing voltage goes up up up. I have seen 15+ volts and the only way to fix this is to replace all the unit's large capacitors.
I can only wish my own finish voltage ended going up up up...
