Build your structure (where possible) in an elevated position with good natural drainage. Place a vapor retarder under floor (gravel or concrete) Ventilate enclosed building when concrete is being poure and continue to ventilate until it is cured. While not a problem on the grass or your car, condensation on metal panels can be a problem. I am fed up of condensation on the inside of a metal she especially the roof. Water vapour carried in the air then condenses on the colder surface of the roof sheet.
Review lighting fixtures, too, as they can have varying effects on indoor moisture levels.
Use a dehumidifier or run your AC compressor. The problem you are describing is common with metal roof buildings in climates with high or mixed humidity with temperatures low . How do you stop condensation on a metal roof ? Then I would use a blanket foil combination with the blanket exposed to the roof sheets , this will take care of . Metal roof condensation , especially with a tin roof, can cause serious problems when the metal. The goal is to use as many full sheets of plywood as possible.
This creates a true barrier which will reduce the risk of tin roof condensation.