This should serve as a warning to people who think storing their vehicle under a tarp is a good idea. Moisture condenses out of the air under a tarp and is trapped in.
Also: place a vapor barrier under the vehicle (to stop the moisture under the vehicle (under the tarp) from rising - from the earth/ground and/or from a cement floor that does not have a vapor barrier under the floor.
An actual canvas tarp is not as bad. But those cheap blue plastic things would be better used for the purpose Tim suggested.
Also, you didn't say whether your cab was sitting directly on the ground or on blocks. The higher off the ground the better, to allow some air to circulate around it.
