These trucks are not modern, really not any more complicated then something from the early 80s.

The KAM on an EEC-IV draws about 1.0-1.5 milliamps of current, your average modern radio only draws around 7 milliamps of current to keep the presets alive. With a 65 amp-hour truck battery, that's 318 days worth of reserve power until the battery is dead, not including self discharge of around ~10%.

I suspect a failing battery or something's using more power than it should.