Not sure what you mean by "dealer lock". There are two locks; one can be set before the door closes (a night latch), and the other is a dead bolt. You should have gotten two sets of keys, with at least two duplicate keys for each lock.
I would think that an Airstream dealer or a good locksmith could make a key for you. Also, I'd call the original owner who probably has a key that they don't know what it goes to. In addition, I'd search the inside of your Airstream to see if you can find one in a logical place, like where the original owner kept a spare, e.g., in a drawer, in a magnetic box attached to the stove hood, hanging on a hook in the closet or overhead cabinet, etc.; or perhaps a key is hidden outside in the wheel well, battery box, around the spare, in the bumper, attached to the television antenna, etc.