Here's the problem:

The truck kills the battery after about 8-10 hours of sitting parked.

Whil I originally thought that the problem is related to wiring mods by the previous owner, I now understand that this drain might be caused by a defective alternator regulator.

Any thoughts on this?

A bad alternator is a lot better to deal with than tracinga wiring problem..

 

Try disconnecting the charge back wire at the back of the alternator and the ground at the battery. Put a DMM at the battery, black lead connected to the battery negative and red lead to the battery cable. Then touch the charge back wire on the alternator and see if it shows the draw you are experiencing.

If that isn't it, you need to go through the fuse box, pulling fuses until you know which circuit is causing the draw, then working from there.

 

Excellent diagnostic!

I'll give that a try!

Thanks!