Best way to secure a trailer?

[tls25rs]

Use a boot.
http://www.jegs.com/i/Trimax/431/TWL...qalGgKSE_D_BwE

This plus one or more of the others should do it

[MarkMarine]

The kryptonite "forgettaboutit" New York 5' chain is the tits. Costs like $150. They also have shackles you can mount to concrete pad to create a lock point to the inanimate object if you like. lock that around the rims and then lock the coupler. The motion alarm is good also.

[Mustang]

Also incorporate a locking cable, but feed it around the leaf spring. This way even if they unbolt the wheel they still have to cut a cable.

I do this. Krypto chain through the leaf and rim.