Assuming you have the owners manual handy:
With a charged battery installed , disconnect the negative battery cable , leave the positive connected to the battery.
Connect a volt meter between the negative battery terminal and the negative battery cable.
You should be reading battery voltage on the meter.
In the underhood fuse panel,pull the fuse for the PCM memory.(On my 2003 it's #21)
Go to the in cab fuse panel.
Start pulling the fuses from the panel until the meter reads 0 volts.
Mark down that fuse number.Leave it out of the panel for now.
Start replacing the fuses one at a time until the meter reads battery voltage again.Each time it does,leave that fuse out.
One of the fuses powers the dome light.My book doesn't tell which it is.
If you remove all of the fuses from the panel and the meter still reads battery voltage,you will have to go to the underhood panel and do the same procedure.
This should narrow down which circuit is drawing down your battery.