I use a long cable or chain to lock the bikes up at night. It's not perfect but I'm sure it keeps most of the thieves away.
As far as avoiding campgrounds that have a theft problem, that can sometimes be problematic. Here in San Diego some of the most popular and difficult to find openings are the beach campgrounds South Carlsbad and San Elijo . Yet these two campgrounds also have a high theft problem. So if you want to take the kids camping on the beach, you have to either put everything away at night, or take a chance that it could get stolen.