I'd think that an annual RLO rubdown would keep dryness away. There are also quite a few gun safe dehumidifying solutions out there, from absorbents to heating elements. I live in a pretty damp area (PNW), and have more trouble staying on top of mold/mildew than anything else. I've found a mold killer that comes in a pump spray bottle that works very well. I've been doing some experimentation with it on old leather and wood, and so far it's been great. I'd be happy to share the name of the stuff if I could only find out what the CO (OC) has done with it.