Have you checked that the alternator is connected correctly? If the wires aren't in the correct place it will drain the battery.

 

Both of my dent sides have small draws that will drain my battery over the coarse of weeks. I don't drive either of them daily or even weekly sometimes. I've gotten to where I've been driving both at least once every weekend. I finally equipped both with battery turn off switches. When I park them for any length of time, I turn off the battery. One truck had once had some kind of auxiliary battery setup. On the front side of the radiator support, was left a battery switch. I wired it in-between my master ground wire and the negative post of the battery. My other truck has a very simple blade connector between the neg battery post and a post that my ground wire attaches to. I suppose I could trace down the draw, but it's not an issue when I drive the trucks on a regular basis. Since I've gotten in the habit of turning my batteries off, I haven't had a drained battery problem. I might slow down some dim witted thieves too.

 

Battery cutoff is nice because it eliminates (well reduces) the possibility of heavy shorts and possibly starting a fire.
If you're like me there's always a hair shirt element to leaving home for more than a few days. Is the stove on, did I unplug my workbench?

Disconnecting the battery is a good way to avoid worrying about unlikely but possible things like rodents chewing on insulation or whatever.