We are building a gravel parking pad/driveway for a small travel trailer. Suppose it is possible that we may want to eventually pour concrete on it, but for now thats not in the plans. Getting conflicting advice on whether or not we need straight 20 ml gravel, or crushed gravel (with sand) for this project. Area measures 12' x 20'and 4-6" deep. Any suggestions or pros and cons of these choices appreciated.