I have a couple of things as mine is out the front, drop the corner stabilizers down as a start and keep a cover on it too, i have a wheel clamp from Repco, a tow hitch lock (Mine is a Trigg hitch) and i also have a steel cable lock that runs through the wheel and over the suspension and axel with a big good quality anti bolt cutting security padlock! even if they get the wheel off its still attached to the axel!
I have also seen people take a wheel off and sometimes the hub too!
I also have this sticker from a large caravan security company.....
I can hook you up with these stickers too, i had them made up.