We use a coupler lock on our Hardside, but also have a hitch and pin lock (came as a set, all keyed the same). We also have a locking battery clamp to prevent it from being stolen.
My co-worker and her spouse were on their way to pick up their boat from storage, stopped and ate lunch. Came out to their truck and discovered someone stole their ball mount and shank from their truck receiver.
We rarely feel theft will occur in a campground. The stories I have heard, have occurred in owner driveways/yards, storage facilities and city streets.
Locks may not stop a theft, but they certainly can deter the thought. I think a thief would more likely bypass a rig that appears to have locks and move on to one without.
2012 FR Flagstaff T12SDTH
1996 Shadowcruiser Pop Up Truck Camper
1967 Newell Motorcoach
2003 Ford F150 5.4 V8 Triton Super Cab
2004 Nissan Titan LE 5.6 V8 4x4 Crew Cab