The Maverick shotgun should be pretty easy to clean.  I haven't pulled one down, but usually the bolt is removed after the barrel is taken off and trigger guard removed (on most pumps).  The bolt will come out through the front of the receiver after the barrel is removed.

 

If you can't break it down, a good aerosol cleaner like gun scrubber will work to blast out the accumulated gunk.  It comes with a long snorkel to get into nooks and crannies.  After you've allowed the solvent to evaporate you can give the parts a light spray with WD-40 to protect them.

 

WD-40 is a nice lubricant, but it will not protect steel for long times (it was not designed for this purpose).  The spary evaproates and the lubricant is easily wiped off.  It is a temporary protection between cleanings.  There are other sprays that will lubricate and last much longer, such as Break Free CLP that comes in an aerosol can or squeeze bottle.