I dealt with this exact thing this year. My "reserved" sign and the traffic cone wound up gone, and there was a tent in its spot with the owner playing coy about how the cone with my sign magically wound up teleporting into a ditch that was mostly uphill from the site.
One of the things I'm looking at buying is a small trailer for trips to places where there are first come, first served spots. Mainly, moving a secured trailer is a lot harder than moving a sign without being observed. Plus, having the added storage and solar panel space doesn't hurt either.