Does the battery go dead while the car is being driven daily, or only when
it sits for a week?

What kind of aftermarket electrical equipment is installed on the car?

A service technician at a dealership will not know (or care) about
aftermarket equipment, and will not be enthusiastic about troubleshooting
a heavily modified vehicle.  (Flat-rate technicians do not get paid very
much for diagnosis, especially when the vehicle is still under
warranty.)

If the battery goes dead while the car is being driven, it's the charging
system.  Tell the dealer to check the generator the old-fashioned way,
with a Sun VAT-40, or if they don't have one of those, a carbon pile and
an amp probe will do.  If the dealership doesn't know how to do that, find
a different dealer.

If the battery ONLY goes dead after the car sits around for a few days, it
could be excessive parasitic draw.

To check the parasitic draw:

Charge the battery completely.

Leave the hood open, but close the doors, glove box, trunk, etc.  My 2000
Maxima turns off the interior lights automatically after a few minutes,
but when you're testing, make sure that you physically switch them off.

If there's a light that illuminates the engine compartment, disconnect it
or remove the bulb.

Turn off the ignition and remove the key.

Disconnect one of the battery cables of the battery.

Connect a milliampere meter between the battery post and the terminal
clamp.

If you're not familiar with connecting a milliamp meter, read the
instruction booklet.  If you're using an analog meter and you connect it
backwards, you will not get a good reading, and you may damage the
meter.  Digital meters, on the other hand, will read "negative" ampere
measurements, at least mine does.

The initial reading may be high when the control modules wake
up, but it should drop within a few minutes when the control modules go
into sleep mode.

I don't know what Nissan's specification is, but when I worked on BMWs, a
20 milliampere parasitic draw was acceptable, but that's still quite a
heavy draw.  When a car was in good shape, the parasitic draw was usually
around 2-5 ma.  The highest I ever saw was 12 ma.

If it's over 20 ma., start pulling  and replacing fuses one at time
to see if the draw goes away.  If the draw doesn't go away, pull the
fuses one at at time, but don't replace them.  Make sure you note the
ratings of the fuses so that you don't inadvertently put a 30A fuse where
a 10A fuse belongs.

Eventually, the draw will go away, and you will be able to locate the
faulty circuit or component.

Good luck!

This just needs some troubleshooting. It sounds like the dealer does not
know how to properly diagnose this problem. You need to go somewhere
that does. It's not rocket science, someone just needs to measure the
current draw and find the circuit responsible.