If this is a recurring issue, then set a trap. Stay in your van reading, or something, with the front curtains closed. If your neighbour then comes over into your site to scope things out, you then confront him. Take lots of pictures. Then either report it to the office, or the cops. Best to do this on a rainy day as you wouldn't want to wreck your outdoor camping time! :)
But really, this should not even be happening in the first place! If i have claimed a site, whether it's in a private park, or public, then i should feel comfortable leaving my items in the campsite! It is the people who come into your site to steal (liberate) things that are in the wrong and should be treated accordingly. Traveling in a van makes it difficult to pack everything up when you want to enjoy the surroundings. We should not be held hostage by scum!