RCPete,

I put a Viper car alarm in my trailer. Its powered by the 12V deep-cycle marine battery I have in there for charging, etc. It will go off if the trailer is moved, or if either of the doors are opened. I don't yet have it setup to flash the trailer lights, but doing that is pretty easy with a 12V relay - as would also be the case if you had electric brakes on the trailer and wanted to activate them too. At $200, it cheaper than the trailer-specific system someone else suggested - which I'm sure works well too. If you put an alarm on the trailer, you'll get a break on the insurance too.

Bob