I store my truck under a "lean to" that is attached to the side of my workshop in my back yard. It currently has two open ends (to drive straight through), two walls, a roof, and a dirt floor. How bad is a dirt floor for moisture?

I am trying to decide between keeping a dirt floor, a gravel floor, or a slag floor. Which do you think is best for vehicle storage and why?

 

a hard pack dirt floor will promote rust. a sand, or good draining gravel floor will drain the moisture off and not rust. slag will also act to get rid of moisture.

 

They all have their upsides and downsides. As previously noted, hard packed dirt will hold moisture and can promote rust. When it holds enough moisture its called mud. Ever work under a truck in the mud?
Sand drains well but has a tendency to not stay in place when you drive on it. Gravel also drains well, pretty much stays in place but is tough on the body sitting or laying on it. Same for slag only sharper corners.
If you have your heart set on gravel, see if you can get river stone at a reasonable price in your area. It can also be laid over coarser gravel, stays in place and has a smooth surface.
Another possibility, and one that is used a lot in my area is recycled concrete dust or quarry dust. If done right, it's the next best thing to concrete after its packed down and is reasonable cheap.