[maple_leaf_eh]

The issue is migration of moisture from the soil. In my backyard (the only longterm test laboratory at my disposal), the "tin" garden shed used to be resting on the topsoil. There was a sill made of galvanized folded sheet metal that crumbled and rotted. As soon as I placed the shed on a barrier layer, all I had to worry about was moisture from the atmosphere.

When discussing how to prepare my Jeep for long term placement (notice I didn't imply storage), the collective wisdom recommended putting a barrier floor on the shelter. The tires would not rest on gravel. The floor is in fact a couple layers of Corplast sheet material. After every Canadian election, there are various sizes free for the salvage. The laws require signage to be removed promptly after the election, so many hands make light work.

So, park your steel tracked vehicle on something that is a positive barrier to moisture. I would not be too confident about concrete. Any cardboard box stored on the floor of my father's garage has rotted through. We get around that by resting everything on a sacrificial layer of wood or pallet.