I don't lock my hitch, just secure the TT itself. I don't think it's very likely that they will steal the whole trailer, they are more likely to break in and steal things out of it.
I take the TV/DVD out of the TT when it goes to the storage lot. Not much left that they'd want to steal. When it goes for the winter the battery, and maybe the gas bottles, come off as well.
I know people that take the license plate off when they store it, some of them put a placard in its place that says "Stolen" but even that won't stop them from taking it.
If they want to take the whole TT, well, that's why I pay insurance premiums.