Hi, and welcome to the forums!
Now to your problem--try posting a picture of your entry knob, as it might not be original, and we need to be sure of what we are talking about before trying to diagnose.
In general, though, look closely at the shaft behind the knob and make sure the shaft is actually turning as you turn the knob. When you say "the knob just turns," does it go round and round, or just a 1/4 turn in each direction? If it goes round and round, and the knob is turning the shaft, then the mechanism is probably broken internally. Contact the previous owner and ask if this is a problem they have seen in the recent past and if they have a trick to solve it. Is it possible to get inside the trailer via any of the windows?
Worst case scenario, the mechanics of the knob have failed internally, and you will have to gain access to the striker from the outside. Again, this will require knowledge of the anatomy of the knob, so a picture will be worth a thousand words.