Hi all,
Thank you all for your helpful suggestions.
We decided to take the camper in to our friendly locksmith this morning. 1/2 an hours work. He took the back off, stretched the springs out as they had become compressed and lubricated all. Then locked himself inside and had the boss try to get in from the outside. It worked. He also suggested putting a small patio bolt on the inside as added protection as we would always be able to lock from the outside but if the inside went again we would be secure.