If you've got a motorcycle trailer (or any other type of trailer), you'll know that they are not safe from thieves. It's easier to steal a trailer than it is to steal a car. Putting a chain and padlock will not solve the problem (as I found out the hard way).
Putting a car or motorcycle alarm on a trailer is not easy. They are not designed for them, so they'll not work the way you want them to. But you can get trailer-specific alarms.
Trailer Dog Security sells several models destined for any kind of trailer. Their top of the line alarm is called the TD6000G.
Apart from the normal paraphernalia which come with alarms (like sirens, flashing lights, battery, etc), the TD 6000 G is solar powered. You can fit a solar panel on the roof of your trailer, which will give it electricity. If there's no sun (or it's indoor), your battery will last several days.
The TD 6000 G will sound two sirens, flash lights and lock trailer brakes if any of the sensors detect unauthorized activity.