I think you will find that most, if not all, stand alone National Park Service locations (National Parks, National Battlefield Parks, and National Historic Sites) have some kind of designated RV parking. Maybe not right next to the Visitor's Center, but somewhere nearby.
Exceptions might be densely urban sites like the Charles Town Navy Yard (USS Constitution; Boston) or Independence Hall (Philly) or anything in DC. On the other hand, there isn't much convenient parking for any kind of car or truck. Public transit is your friend.
