Are you saying that the van is now an empty shell - all metal? The moisture has to be coming from somewhere, and you have to find where it’s coming from.
If it’s a simple issue of water from the humid air condensing on the cold metal surface, you’ll have to install insulation in addition to installing vents.
Exhaust vents are typically installed in the roof, or at least up high in the side. You also have to provide a way for fresh air to come in. In my van I installed the roof vents toward the back, and installed an air intake in the floor toward the front. There has to be a way for fresh air to come in. Can be an open window or open vents at the opposite end of the van from the exhaust vents. Exhaust vents toward the top and intake vents as low as possible, opposite end of the van.
In the winter, my heater (solid fuel heater) draws the air in through the floor vent, uses that air to burn the fuel, and exhausts through the chimney (flue). The exhaust vents stay closed to prevent drafts. This type of heater dries the air inside the van because all combustion gases are exhausted through the flue.
In the summer I leave the rooftop vents open, most of the time not powered on, and the air enters the floor vent and goes out the roof vents. This is due to the natural tendency of warm air to rise and escape through the roof vents. As long as there’s a way for outside air to come in (in my case the floor vent), outside air constantly flows through the van keeping the inside about the same temperature as the outside air. On hot days when the sun is heating the metal of the van and coming through the windows via greenhouse effect, I accelerate this flow by turning the fans on.