I understand the OP is concerned of outdoor moisture from his gravel driveway.
I advise to make sure there is plenty of air flow under the rig. Don't store things under it, don't block it with anything, not snow either. That will allow air to flow under the rig as much as possible. Laying down a plastic sheet and driving on top of it just might create more trouble than help, unless you are anal with it.
As far as the interior is concerned, I agree that a dehumidifier is a good idea. The only concern is if the collected water freezes, then leaks out of the cracked reservoir or drain tube, then does worse damage inside.
You got some other interesting suggestions, keeping it warm inside, opening all cabinets and fridge to help keep it more dry inside.