both wires must run from panels to controller and NOT be connected to anything else. they should be fused.
I know using the chasis for negative is commonplace for 12v systems. Doing so will add additional impedance in the negative leg of the controller causing problems. remember that thing you just calculated, voltage loss? Steel doesn't conduct electricity nearly as good as copper. In autos, I don't think there's a 30' run anywhere, so loss will be minimal.
If you run your copper wire to the battery, you are setting yourself up to install a shunt for a battery monitoring system. On those systems, positive current is not measured. Only the negative side passes thru a shunt. All negative current must be measured.
here's a basic schematic of my system. Take it as a guide, as there have been some corrections/changes made since I drew it up.
dang it, it posted upside down, sorry.