It's for a group of two. If you park on a spot for doubles by yourself you will be charged for the two sites.
That's a good question. You should call, but it might simply mean double the amount of people can stay there. Are you sure the site is for RVs, because sometimes they have "double sites" to state that two cars can fit. Which would also make sense why it would be listed as an RV site, as well. Sorry I couldn't be of more help.
You should call them and find out
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