If the tools aren't being used for large periods of time that coincide with the biggest problems, it seems the biggest help might be to remove them from that environment during that time period.

If the issue is mainly humidity over temperature (my problem is high humidity in my house settling on things when it hits the back room where it gets colder because we're too cheap to heat the house much - while it gets humid in the summer, it thankfully hasn't been much of an issue for rust if I keep things cleaned of dust and fingerprints) I wonder if a dehumidifier would be a helpful, and possibly more effective than an AC type unit? I haven't any experience with these outside of the one my parents have had in their basement forever, and living in VT, obviously my climate issues are much different than yours!