[mr48chev]

The starter won't drain the battery.

Have you checked the output of the alternator or generator to make sure that it is charging what it should? You may have a charging problem rather than a drain problem.
If you have a big sound system or high powered halogen lights the alternator may not be putting out enough amps to run those and charge the battery too.

A battery drain is usually caused by one of the following:
Something left on like an interior light or glove box light

Blown diode in the alternator that is letting power drain off.

A dirty battery top on top post batteries. Take your volt meter and check across the battery on different spots on the top of the battery to make sure that the dirt and moisture isn't letting current flow across the top of the battery. I've seen a battery go dead in three hours due to a 9 volt drain across the battery before

Someone in the family who goes out to the truck and plays the radio or CD player when you are gone. One of my students used to leave his lowrider at home while he worked at the local grocery store and his battery was dead too many times in the mornings. After jumping it and checking it in the school shop more than once his mom says "oh yes, your brother and his friends play your car stereo while you are at work real often".

Is there anything that you added to the truck that might draw power when the truck is parked? An amp that is wired so it has power to it all the time? A security device?