First, the only way to get a reliable indication on battery health is by fully charging (best with a multi-stage charger) then disconnecting them completely for 30-60 minutes and then check the voltage.
Second, relying on you vehicle to recharge your batteries will NOT get you a full charge. Running the generator will (assuming your converter works well).
Unquestionably 4 year old batteries are past their prime. If I was going to replace them I would buy two 6V golf cart batteries from Costco. Best bang for the buck. You are not running any big loads (no big inverter) so I think your existing wire is adequate but remember the 6V batteries have to be connected positive to negative.
