Sure they can. It makes no difference what kind of hitch you have. If someone wants to steal your trailer, they WILL. But the fact is, it doesn't happen often. What happens regularly is they break into your trailer and steal your belongings, even in "secure" storage facilities. Or they steal components off your trailer. In your case, I think I would store the trailer without the $2000 hitch attached.
curmudgeon: noun; a crusty, ill-tempered, irascible, cantankerous old person . . . .
79 27' Holiday Rambler Statesman, 78 32' HR Imperial, 85 36' HR Imperial 5th Wheel