It doesn't take much play to cause visible sag.
You can raise the door, by placing a jack under the bottom edge— well padded— and jacking it up slowly, one stroke of the jack handle at a time. Watch for movement around the hinges; you'll probably see some at the upper hinge.
The jack can also help support the door as you pull the hinge pin. The pin will be a lot easier to remove if the jack, and not the pin, is supporting the weight of the door. I think you'll find that the pin is worn, and no longer the same diameter as it used to be. Either that, or the holes in the hinge where the pin goes are larger than they used to be. Or both. If the two leaves of the hinge are both tight, on the door and the frame, then the problem can only be the pin.
Given that the trailer is over 20 years old, and aluminum is a relatively soft metal, I'd be surprised if the hinge pins aren't worn.
I thought getting old would take longer!