take it to the parts store for a charging system checking. sounds like your alternator went out or is workin intermittantly.
a fully charged battery at rest should read 12.6 volts. i dont remember specs, but i think i am at 14.5 volts at 1500 rpm.
if you are at 12 volts even (i.e. 12.0), then as soon as you turn on an accessory, you will be discharging.
you could also have a bad cable from the alt to the battery (not allowing juice to flow). you can check alt output (test at the alt). compare to what the battery shows. if there is a large voltage drop, that could be it.
also if you have a dead cell on the battery, that can keep the juice from the alt from being stored in the battery.
parts store free check up should tell the tale. charge up the battery first and have a friend chase you in case it dies out. better would be to pull the battery and alt and have them bench (load) tested.
good luck
You also may have a draw on the battery. Take the neg cable off put a test light between the battery and the cable if the light comes there is a draw. have someone pull out each individual fuse until u find the circuit. Then it should be pretty easy from there to find out which component it is.