Almost certianly a drain.
Most parts places can simply test your alt, and tell you if the diodes are bad in it. I would be willing to bet money, it's your radio. Every single time I've encountered a drain, it's the radio has gone bad.
The simple quick fix. Go to harbor freight, and spend the $5 (it was $15 for me, as I have AGM batteries) for a float charger, then install it in your truck. Then simply plug your truck in if you're not going to drive it tomorrow.
My truck sits for weeks at a time, so this was worth every penny for me. Now the shedevil starts quick, the first time, every time, because of this.