I would tell you that it is safe to leave it there. I live about 5 miles from the grounds and have never heard of a lot of issues with the parked campers. I have never done it before, but people have been doing it for years and for the most part it is a pretty good type of people out there.
When you buy a campsite you get one pass for your camper(or tent) and one for one vehicle. I am not sure if you can purchase additional vehicles passes without buying another campsite.
They do have water around the campsites, but I usually bring my own bottled water just for the convenience. The spigots on the grounds can be a little walk depending on where you set up.