If it is just a matter of the key not working the tumbler can be replaced for less than 20 bucks.
A deadbolt can be added but requires some modifications to the bolt to minimize the hole in the door frames. A standard hole will leave you with on frame left
The lock was made for Airstream by a 3rd party. At this time I believe the only way to get one is to get a rebuild and they are not cheap. There is a way to retrofit a current style lock in the door, but a filler plate is required and it will not look good unless you are ready to paint.