When you say you tried another battery, was that second battery charged up, and what’s it’s condition. Even a brand-new battery fresh from the shelf of a parts store can be low on charge and may need a few hours on a charger before it’s ready to start a car, especially one with a large engine like a Holden Statesman. If the battery was just another one you borrowed from somewhere else, it could well be about as worn out as the one you replaced. Even a battery that shows 12 volts when you test it with a voltmeter, can suddenly drop its bundle when you place it under load (such as trying to start a car with it). You need to check the battery’s current while it’s attempting to turn the engine over.

Other possibilities could be worn out battery leads (do they get hot when you try to start the car?) a poor earth somewhere between the engine and the car’s chassis or even battery terminals that are old and crusty. Don’t rule out a failed starter motor, either. Often a worn-out starter can pole out internally, causing a huge internal short that will produce the same slow cranking we associate with a dying battery.

THE V8 Adventra was a bit of a dog in many respects, but 20 litres/100km is a bit too high. The Adventra is a heavy old nail so I would expect it to be pretty thirsty. I would think it should get 16-17 litres/100km on average. The only thing you can do is to make sure all is well with the engine and the electronics controlling it, make sure your tyres are inflated correctly and that wheel alignment is correct.