Is there a wheelchair narrow enough to fit through the doorways of a small trailer?

Of course there are. You can get wheelchairs of any width. There are wheelchairs that are built to go through the aisles of airplanes and they are very narrow. Without knowing the width of your doorway and the width of the person who will use the wheelchair it is difficult to advise you.

The narrowest wheelchairs are called transport wheelchairs. These are different from standard wheelchairs in that they have smaller and narrower rear wheels. If you are looking for the narrowest of wheelchairs start there. You can read more about the difference between transport and standard wheelchairs at the article below.