When my 2002 Nissan Xterra sits too long the battery goes dead

Battery will drain if the vehicle is left for weeks at a time. Things like the anti theft system will use power ALL the time while sitting. Driving it daily wont let the battery dicharge because it charges as you drive, A better battery with hight reserve capacity might be al;l you need, not a cheapo Chinese import from walmart... Or it may have what's called a parasitic drain. Search for it like this, and wait about 1 hour after hooking up your meter for the modules to sleep. Get an analog voltmeter remove the negative battery cable, attach the Black lead to the battery post, Red lead to cable end, then pull fuses one at a time, make sure there are no accessories or lights in the on position, Key off! Close the door have someone in the car, pull fuses one at a time after each fuse check meter, when you pull the fuse for the bad curcuit, the meter will drop to 1 volt or so. Check all fuses, even under the hood. If meter does not drop, remove the battery connection from the alternator, if it drops now, the diodes are bad in the alternator. Put the fuse back in were it goes as you go so you don't forget which fuse goes were! Wait several minutes after hooking up the meter before pulling fuses, test with a good battery!
https://www.2carpros.com/first_things/why_is_my_car_battery_dead_over_night.htm