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Sounds like you have a current draw on the battery. Something in the vehicle is needing too much power and is causing the battery to die. Draws unfortunatlly are the hardest to find problems in any type of vehicle. A volt meter will need to be hooked up in amp mode to the batttery in a way that it can test for a draw. After the meter is hooked up every fuse, one by one will need to be removed from the fuse box's to determine what is causing the draw. This is a very technical diagnosis and requires a fair amount of skill to do so. If you think you can do this and would like further information on how to do so, Please let me know.
You will need a volt meter that has an amp setting. The black lead from the meter needs to be in the ground port or common port on the meter. The red lead needs to be in the amps port on the meter. The meter will need to be set at 10 amp max with the dial. Remove the positive battery cable and take on lead an secure it to the battery post and take the other and secure it to the battery cable. The meter will work as part of the battery and evrything electrnic will stil work in the vehicle. After you have everything hooked up, Wait a few mins for the computers to go to sleep. Then start by removing the fuses one by one. If nothing changes on the meter for your reading, You knwo thats not the draw. If when you pull the fuse and the number drops, Then you have found your draw. You will have to do this for all fuse boxes in the vehicle. Make sure not to open any doors, or turn the key on at anytime as this will screw your readings. You may have to put the window down before you start so you can crawl inside to gain acess to those fuses. It can get quite confusing for the average person.
Here is a generic picture of a meter and what you need to do with the leads.