One other thing to consider is condesation, it's a big problem with sub-zero eweather snow camping, mostly noticeable in trailers without "mouse fur" on the walls. Just 2 people breathing causes enough moisture to freeze on the windows into a layer of ice...not to mention cooking, bathing or more people (family ?) & everybody's wet clothes in a small trailer like a GT.
Most Airstreams especially vintage ones for the most part are considered 3-season trailers in thier construction - some of the moidifications you have made will help - but the insulation will probably be lacking.
Good luck - let us know how it works out for you ~