Is the exterior hinge such that it can be removed easily? I know our trailer's hinge pin can be removed without damaging the trailer or door.
Another possibility, do you have a "baggage compartment" that you could get a small child into the trailer? Both our Airstreams have had exteriro compartment doors that go to spaces under the bed...a 7-8 year old could crawl in, lift the bed and open a window (or the door from the inside) so you could gain access.
Sounds as though you will be needing a new door knob regardless...