Going out on a limb here... Are you using the correct key?
A while ago I had a similar situation. Was all set to try to gain access from inside the camper, gently pry open, disassemble, etc...
...then I realized I had inadvertently grabbed a set of spare keys that I had on hand from an older previous camper. Those keys are so similar that they will fit/ insert into many but not turn. Once I got the correct set I was in.
I am a fan of the KISS principle and this is the first thing I can suggest before more drastic measures.