You need to see if there is something draining the battery. Put an ammeter in series with one of the battery cables and its battery post and see if it draws any current with the ignition and everything else turned off.

If it is drawing current start pulling the fuses one at a time while observing your ammeter. When the current drops after pulling a fuse that is your leaking circuit. If after pulling all your fuses and you are still leaking current, get a wiring diagram and start looking at the other circuits that are hot all the time and not fused in the fuse box such as the starter solenoid, heater blower motor, glow plug circuit and whatever else you can find in the wiring diagram.

Good luck,