Hi Ken 

Just a comment, it might be better in the future if you post this type of question in the technical threads. It might get better answers there perhaps, and you might be able to find answers there by looking at previous threads too.  

But to answers, firstly the battery will not be overcharged by the car. However if your Commodore has a 'smart' or variable voltage alternator, which is what I expect it has, then the battery will not get charged well at all. To charge a second battery with modern cars you do need a DC-DC charger fitted near the battery and supplied by a generous sized cable and plugs from the car battery/alternator.

Secondly there is a Victron meter which will do exactly what you asked for. Victron BMV700 series. It will read the power going in and the power going out and tell you how much you have in the battery to use.  https://www.victronenergy.com.au/battery-monitors/bmv-700 Read about them there and the different types. They are available from all suppliers who sell quality solar gear, including from ebay sellers. The unit needs to have the setting entered initially to suit your battery to be accurate.

Jaahn

PS go to the Techies Corner for more ideas and also the Solar Power sub forum at the top of it. These things have been discussed there. While you can use a VSR as previously said by johnyboy on older vehicles(i have one) it will not be satisfactory on newer vehicles with variable voltage alternators.    Also if your wiring is not good enough with a VSR the voltage losses in the long cable runs may restrict proper charging.