Concrete or asphalt would be the best to park it on as long as there is a "crown" or angle that allows water run off.
Dirt/gravel is a poor choice, moisture comes right up through the ground and gravel.. Lots of vehicles rot from the underside very quickly when parked on dirt/gravel and don't get moved often..
Your over thinking this, simply open a few windows slightly (even sliding windows can be opened slightly without fear of rain getting in) which allows the air inside the RV to be exchanged with the air outside the RV.. Inside of RV will not get any wetter or damper than the air outside..