Storing it in your truck is the problem....
and like others said ...condensation is the enemy ....
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Any time the air is humid ...when its warm, it'll hold moisture. As the air cools off...it wants to get rid of the moisture...and it will condense on the coldest thing around ( windshield, cast iron, steel...)....
I'd store the gun in your house ...or you're fighting a losing battle. Even in your house it'll be an issue ...unless you can keep the temp from dropping in the area where you store your guns ( like keep a small wattage light bulb on in the safe, or a golden rod, or something). Make sure the gun is as dry as possible when you store it ...and spray it with a light coat of oil ( like Rig #2 oil or something ) ...but you'll still have to check it often.
changing coatings might make a small difference....but you can't change the way the air loses its moisture as it cools down at night.