I don't know if you've looked into it, but you could spray it with corrosion x maybe?
what part of norcal are you storing it?
do you have any local RV builders who could check up on it every three months or so?
I don't imagine any desicant would be useful over the long term, it will get saturated and stop working pretty quick with the typical humidity of CA. eh, i saw it would be in a sealed container. You might be able to get some pretty big desicant that would make a difference.
i tried to contact this guy:
http://www.glocktalk.com/forums/show...php?p=12512521
but i didn't get a response, he might have something you could use. maybe you could scrounge some of this stuff from the air force.
I think the plastic is a wash, it might hold some moisture off the flat surfaces, it might hold it on the edges. It will get harder to take off eventually.