Does your TT door have a deadbolt (with key) in addition to the regular lock? If so, use it. The regular door locks are not individual to each trailer, so anyone with a key to another TT of same make could get in that way. Deadbolt locks areindividual to each trailer, however. At least, that's what our dealer told us. Whenever we left our TT there for service, he asked us to unlock the deadbolts but leave the other locks locked, since he had a master key & could get in if need be.
Another possibility is that someone is pretty good at picking locks. :(