Powermanagement of a minicomputer in an automotive vehicle by engine start/stop and voltage threshold detection
What you are attempting sounds backwards.  You want to detect higher than idle voltage, not lower.  When the engine is running, the alternator raises the battery voltage to the float charge level.  This is usually around 13.6 V, but can vary quite a bit.  A threshold of around 13 V or a little higher is probably right.  Measure your battery voltage with everything off and with the engine running, then pick a voltage in between.

The rest comes down to detecting this threshold voltage using very little current.  Something like a TL431 and a couple precision resistors may be good enough.  Carefully compute the error band to make sure it is acceptable.  Then turning on power to something else when the TL431 triggers is trivial.