+1 on telling them they can have your camp site (and sort the costs out with the campground) and get a new one for yourself. It would be a good idea to check with the campsite anyway, the charges may be different for a tent+bike versus a tent+bike+someone else's car.

If they insist, and you don't want to tell them to shove it, charge a fee for the space. If the cost is higher because of their car, make sure they pay the increase in full and then half of the remaining cost.

--J