tires are made from rubber, rubber will break down over time. Ever see a bicycle tire or old car tire that has aged? -Usually cracked and brittle.
Zip bags are a good idea, some of the SCT tire manufacturers sell their tires in re-seal able bags (I have some, they are great for storing your tires).
What is "Tire-Tweak"?
Also, I am not sure what effect it would have on rubber RC tires, but medical grade Glycerin is pretty amazing stuff, especially for healing aged rubber. I've used it on old 1:1 cars, seriously, for rubber trim and molding that has aged it will bring it back to new. I've used it plenty and it works wonders. It doesn't seem possible, but if you apply Glycerin to aged rubber and reapply over the period of a few weeks as it absorbs into the rubber you will be able to bring rubber back from the dead. For our RC tires not sure what this would do though in terms of traction ability. And for $30 I'd just go buy a new set of RC tires.
Read down a few posts, classic car guys such as myself use Glycerin all the time:
-be warned you'll get weird looks and lot's of questions asked when buying the stuff. It has to be pure Glyrcerin, -much of the stuff at the pharmacy has other ingredients and perfumes in it. You don't want that stuff.