The additional information helps. Since your properties are the "business" (licensed campground) you would be justified in claiming the gravel costs as a maintenance expense. Your personal use would be incidental to the business use, the opposite of my earlier conclusion.
From personal experience with a 3/4 mile gravel driveway, additional gravel is periodically required to maintain the road's ability to handle traffic, or to handle increased traffic. This appears to be the case in your situation.
