Our Ford minivan has a "trailer package" and that includes a separate fuse panel for the towing harness. NOT shown in Owner's Manual! I went to check but would have to take more apart than I can get to right now. I remember it having several fuses in there, and I want to say there were Left and Right for Stop/Turn.
A Class C Chassis doesn't come with Rear Lights, but it has a Trailer Harness. Although the wiring is still there, often the Coach is connected to Trailer, not to Chassis. You don't say which Chassis you have or the Year, so we can't say what you might have. Check every possible fuse position as well as checking wiring and ground.