what kills a battery overnight on a 84 300zx

These cars are notorious for electrical problems. But you havent provided any information. It could be a million things.

charging system is charging 14v but after sitting all night battery is dead,no interior lights on, all switches are off

also randomly won't start or turn over, dash lights, radio come on but won't crank, not the starter. Seems to be some type of relay problem, no sound at all when key is turned

was the anti-theft relay switch.....;)

i have the same thing with my Z but what i do is but the the blinker on when my car is off and the battery wount die try it out yeah i have looked in to it too it i the anti-theft what happens is that it uses the power and when the power gets low enought the head lights and stuff goes on so what i do is just leave the blinker on when i turn off the car and i have no more problems.

timer control unit shorts out taking out the dome light,security. and causes the dome,security,lights in door locks to stay on all the time, or not work at all.this timer control is beside the heater fan box and box removal is required to access it ,the timer is behind a rubberized tape in a recess in the body of the car.