Battery, Alternator, or something else.

[CT-Truckster]

ok when my truck is idling the volt gauge shows good well over the 13 mark, but the more electrical stuff i turn on the lower it gets. like at night with headlights, that brings it way down, and so does the heater fan on high. and when the lights are on i can tell they dim noticeably when turning the fan on or something, they just don't look as bright as they should be. the needle will get way down on the gauge in the bad zone with accessories on, but with nothing on it shows good on the gauge. the belts are all tight. the truck starts good, not like the batt is weak or anything. except for yesterday when i left the lights on for eight hours whille at work. the clock in the truck stopped at 11:30 and it was about 6:50 when I got to work and cut the truck off, so thats what, about 4 1/2 hours till batt went compleeetely dead? i got a boost and its just like it was before. so what do you guys think? batt, alt, or some other unidentified major malfunction?
ohh and the gauge will pick up when you get up some speed goin dow the road.