metal Pole barn Humidity

Dear Experts, I have a metal pole barn/ 3 car garage that's about 10 years old. I have two classic vehicles and a 2005 pickup inside it. The roof and sides are insulated and I'm not sure if there are any ridge vents in the roof, I will look for my contract to see if they were included. I have never noticed any moisture inside the building or on my cars at all. Recently I noticed rust underneath my truck on suspension parts and inside the engine compartment...mind you this vehicle is rarely drive and only has 1,000 miles on it. I have a Temp/humidity gauge inside the building that said 64 % humidity. What if anything can I do to lower the humidity in my building ? I plan on putting 6 ml plastic sheeting down under my vehicles. Should my next step be vents or a dehumidifier ??. Thanks for any information.