Gil, I think in an RV application, the center is supposed to be used for backup lights. Since the T@B does not have backup lights (at least as a stock option), it just sits there doing nothing. It's kinda like the pin that controls the electric brakes. Since my T@B does not have electric brakes, that pin is just there doing nothing, but causing no harm. There are several wiring diagrams for the 7-pin on etrailer's web site that shows the various possible uses for the center pin. You can check it out here. https://www.etrailer.com/faq-wiring.aspx
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https://www.etrailer.com/faq-wiring.aspx
Larry & Booger - 2013 T@B, 2012 GMC Sierra
Happy Trails Y'all