Upgrade trailer tongue lock? Chain through the wheels?

I use both a tongue lock and a tire lock. The tire lock is much harder to defeat.

Also, pull the license plate, if they steal the trailer, there is a chance that a cop will see a trailer with no plate and stop it. More than likely not but its a no cost option.

A rather long chain on the axle lets say is also a warning to the cops if you should be lucky.

Good luck.