Probably want to double check that the city you're in allows that. We had a guy buy a retired watchfire demo truck, thinking he could just park it and advertise, the city didn't like that idea.
That being said, and depending on the size of signage, you'll want to either sand bag it down or add some outriggers, that trailer doesn't weigh very much to begin with. You'll probably also want a removable tongue, to deter theft.