Check page 5 of the FSM under fuel system.
Hopefully this link will work:
http://www.nicoclub.com/FSM/QX4/2002/FE.pdf
From the look of it, the rubber section that the filler assembly goes into is actually part of the tank.
I seem to remember thinking that way when I replaced my filler assembly.
The leaks are usually at the top of the neck. There was a recall on them for R50's up to 2001.
Unfortunately, the 2002-2004's are exempt.