Ben, as an RV driver/owner I recommend this route to the track. Take I81 past inwood and get off at Rt7 Eastbound. Take a left at Crums Church, there will be a nice 7-11 on your right (newly built), take that to the end, and make a right, that road turns into hardesty road. There are a couple of 25mph sharp corners but even that long of a rig you can clear it even with oncomming traffic. Take Hardesty to the end and make a right, just over the hump/hill are the entrances, you want the main entrance which is your second right.
Alternative would be 95S-695W-70W-340S, through harpers ferry and Charlestown, and make the left onto Race Track Road and take that 7 miles to the main gate on your left.