I had a 2003 T139. (Hence my screen name.) If I recall correctly, we had a porch switch AND a mystery switch labelled AUX that did nothing. It was probably part of a standard switch panel -- the AUX is often used for slide-outs, which we did not have.
And exactly the same is true of my 2012 X-139.
And if anyone knows of a manufacturer making a new small trailer like the 139 series, let us know! Someday soon, my "new" trailer is going to wear out from over-use.