The Airstream emblems on your trailer or made from cast aluminum and are put on with anchoring pins at the factory. My service manual doesn't say anything about how to get the letters OFF, but regarding reinstallation, states, "Where access to the rear of the cast lettering is not possible, break off the anchoring pin with great care and attach the letter back with blue No. 30 pop rivets. Be sure and seal the perimeter of each letter to prevent leaks."
Hope this helps.
P.S. Why are you having to take them off?