Another in the department of "It-may-not-stop-them-completely-but..."
Most trailers have stabilizers on them and we'd have them down while in camp. By drilling a series of holes along the length on one or two of them, probably the rear ones, you could put a lock through it and make it so it couldn't be raised while the lock was installed. The holes shouldn't compromise the strength of the stabilizer much since most of our trailers aren't heavy-weights. You could work out a series of holes on most any kind of stabilizer style... the holes wouldn't need to match the exact length each time they are set, but just keep it from being raised very far.
It won't stop a thief but added to several other steps mentioned here: hitch locks, wheel locks, tire removed, etc. it adds a bit more "ass-pain" for a thief to deal with.
Hard to pull a trailer down the road with the stabilizers down?

EZ-Hauler 7x12, single axle, V front, sloped roof, all-aluminum, barn doors.