I park my rig on gravel for long periods of time without moving. No problems.
I am just ready to change out a set of tires because of age after eight years on gravel, moving the rig perhaps twice a year.
Any water that gets on the gravel just drains down to the ground, leaving the tires high and dry, except perhaps for a wet spot on the bottom of the tire tread when the gravel is wet, but with no consequence.
I don't use boards between the tires and gravel.