How to Secure Travel Trailer at Campsite?

The way I see it anyone out to steal a trailer has thought it through and will be equipped -- truck, hitch, bolt cutters, battery powered grinder, etc. It's not an opportunity theft like a bicycle or bbq. So all you can do is the coupler lock and things to slow them down like wheel chocks, stabilizers down, etc. A cable lock through a wheel or bumper if you are really paranoid.

Or make friends with the neighbors and watch each others' stuff while out.