Re: Govt. tarps for temporary storage?

Cocooning a vehicle under a temporary roof is often a practical approach, but I wouldn't dismiss tarps out of hand, even in high humidity environments. Tarps have the advantage of very quick application, and if applied correctly can last ten years or more (presuming you spent the money for a high quality tarp).
Trapping moisture under a tarp can be a problem depending on local climactic conditions. One solution is to add vents on the tarp material. Another option is to cover only the roof and then some portion of the carbody (preferably down to the window sill), allowing air to circulate through doorways, etc. but still keeping the weather out.
Putting it inside a building is the way to go, but as we all know this option is not always available. Tarps and plywood roofs are still only interim measures, and each with their own set of difficulties to be sure, but they're a lot better than taking no action at all.