For much of north america, that's a relatively "low" "relative humidity" data point laurens! Typical summertime humidity readings in the afternoon, in the south central and south eastern u.s. Can be 70%>90% during seasonal micro-climates.
That's not what rots this stuff, it's accumulated road salt-type solutions used in much of the u.s. During winter conditions. And similar problems exist in coastal areas, particularly adjacent to the gulf of mexico. In parts of the u.s., folks can still drive inna limited fashion on the beach area adjacent to salt water and the really creates a problem...along with elevated relative humidity levels.
Have you been able to "power wash" the entire vehicle since you brought it home? If not, that would be something good to do, that way ya can attempt to flush all the accumulated road debris out of the crevices throughout. Scout II are terrible about trapping debris inside fenders and rockers. But then...based upon your pictures we've seen, you already know that!
Recording daily rh levels all across the u.s. (real time measurement taken with a electronic hygrometer, not from the weather service) was part of my former occupation. I had 12 years of accumulated data that was lost to posterity when my former employer was taken over by valeo (I know you are familiar with valeo!!!).
Ya May also be able to rig some sort of thermostatic-control electrical resistance heating unit in your shop/storage space, we do that here for keeping moisture levels inside of recreational vehicles under control.