The reverse signal may come from the Powertrain Control Module (PCM), which is a computer, rather than from a sensor on the transmission. See the attached diagram, for my coach, built on a 2007 chassis. You can pick up the signal at the under-dash fuse block ("CJB") or in the Ford F81D-14A318-AC pigtail.
You should add a "signature" to your profile. It contains whatever info you want; most people put info about their coach there, so they don't have to type it into each message. Instructions for adding a signature are here.