This is quickly turning into a 1% vs. 99% discussion. No one has a right to a campsite. If you wake up Friday morning and decide to go to a campground for the weekend, there's no reason to expect that there will be one there waiting for you. There's no difference between someone renting 2 sites and using one and someone renting a site for the summer, dumping a trailer on it and only occupying it on a few weekends.
I understand the OP's issue because he/she is from the northeast where personal space is for some reason sacred. An example: The riders on the commuter rail into Boston have been complaining that there aren't enough cars on trains because the people getting on at the last few stops have to stand. Meanwhile, the middle seats in the 3-seat rows go unoccupied or someone has their jacket/laptop/briefcase/purse on it.
When I read the first few posts, I knew this would be a contentious thread. Oddly, the thread with suggestions on what to put on your site's picnic table to discourage neighbors got a good chuckle from everyone. The purpose of both threads was the same.