Francis - If you're checking this thread, you've just gotten some good information regarding the purpose of that switch. It would be nice if you'd respond and tell us if any of these posts helped.
I like the idea of it being a "Front Door Light", or, the Water Pump switch. My front door light is not labeled, but it is by the door. All the other switches are labeled in my trailer.
Hi-Lo 1707T - Tire Minder TPMS on Tow Vehicle and Trailer, 300W Solar Battery Charger, Equal-i-zer WDH, Progressive Dynamics Converter, Fan-Tastic Fan
2014 F150 Platinum 4x4 3.5L EcoBoost SCrew