1995moose, reading back through the replies I saw your comment on parasitic drain and corroded cables.
Corroded cables increase resistance in the circuit, they do not cause a constant drain when everything is turned off. Same idea of a valve in a water line/hose. The more the valve is closed the higher the resistance and reduced flow is the result. As resistance increases in electricity, voltage and amperage usually reduce. Constant power devices are an exception.
Starters are classified as constant power motors - they will use any combination of voltage and amps to operate. Throw in resistance with corrosion and the available voltage at the starter drops. This causes a higher amp draw to compensate. You get to a point that the wear on the starter solenoid contacts is excessive and can weld them together or pits them so there isn't current flowing through them.
Parasitic draw - basically something in the circuits isn't powering down into sleep mode or is stuck in the "on" state or electronically failed to an "on" state. This allows current to flow and over time will drain a battery. The amount of drain determines how fast the battery discharges. It can range from milliamps to amps. This is one time less is better.