The reason for the condensation, is that there is no waterproof membrane between the camper and the FLOOR.
IF, there was a plastic water proof membrane placed under the concrete floor, you wouldn't have this problem.
So, it doesn't matter what you place on top of the camper, the problem is coming from the bottom.
Try putting a tarp underneath for a while, then check the difference.
Of course placing a tarp underneath of you store it outside, will not solve your problem.