Security thingys
Works better on car trailers as they are lower, but could be done for any trailers really.
Get a serious padlock and chain. Something that normal boltcroppers won't touch. Then under the trailers home with the trailer out of the way, put a serious ground anchor and chain the trailer on the underside. What you are aiming to do is both make the device hidden and make it so good there is a serious possibility that the thief's tow vehicle is badly damaged.
Although insurance companies often insist on a hitch lock, a serious thief will just attach his own and grind yours off at his leisure later. If the insurance says you have to have one, fine, but be under no illusions it won't do that much.
Storing the trailer without wheels also helps, but not a lot as most trailer PCDs are the same and unladen, a four wheel trailer really only needs two wheels and a thief could just slap some spare on.
