I assume your 3170 is on a Ford E450 chassis. Check your Ford Owner's Guide for fuses and relays. In the guide for our 2010 year model, the fuse and relay panels are under "Roadside Emergencies". The 7-pin connector is fed from the fuse panel under the hood. There is a fuse AND a relay for each pin. Look for the fuses and relays with the name "Trailer Tow ...". I think the ignition switch has to be in the "Run" position to have power to the "Trailer Tow Battery Charge" pin.
The reason they use a relay and fuse for each service instead of just tapping off the chassis taillight circuits is that if there is short in the trailer (or in this case, coach) lights it won't damage the chassis system. In addition, they don't have to use switches and wires with high amp ratings to serve both the chassis and who knows how many trailer lights.
2011 Sunseeker 3100
1997 Pathfinder Toad