The negative wire of pretty-much everything, including the solar panels
go to one common ground, usually an engine
block or a common bus just before the engine
block, so you are on the right track there.
circuit runs through the positive wire. The best option if you have two battery banks and you need your panels
to charge either/or, is to buy a Voltage Sensitive Relay, like this one...
They are around $80. You connect the positive output from your solar-panel charge controller to one of the battery banks, usually the house bank. You can do this at the permanently positive side of the battery switch if it suits. You then connect the VSR with one positive wire between the two separate banks. When the battery bank is charging, it senses the increased voltage and automatically opens the circuit so that both banks can charge. When the charging stops and the voltage decreases, it closes the circuit so that the battery banks cannot discharge into each other. If one of your banks is already charged, the depleted batteries
will automatically accept the highest rate of charge.