It is important to note that while airstream changed very little over the decades, there are some substantial differences with the doors.
The helpful video above is good for late-model airstreams, but is not at all helpful for vintage trailers such as the 1973 trailer of the OP.
The door on many vintage trailers especially of the 70s vintage is constructed completely differently.
There is a single large hinge halfway down. Most of these doors are suicide doors which is a whole Nother thread. Overtime this door may shift a little bit due to some of the nuts working loose. If you open your door check the tightness of the nuts holding the hinge to the shell. If they are loose you might be able to manually straighten and align the door with the doorway, and then retighten these bolts. Be careful not to loosen the nuts so that the Bolts fall away inside the wall. Once you have these adjustments made you might want to put some Loctite on to the locking nuts.
Many of these doors have a cast aluminum frame. Under no circumstances try to bend this door into shape because you will crack and damage the cast aluminum Door frame which unfortunately appears to become more brittle with age.
The fit of the door on my 1969 Caravel
varies depending on how level the frame is and how much weight and pressure the leveling jacks are cranked to carry ( it should be none, but if the frame has already been tweaked and torqued out of shape you may have to adjust the leveling jacks so that the door frame fits the door)