In many of the Forest River pop-ups (including A-frames) the switch is located in the driver's side front corner on the lower frame. When the roof is lowered, the plunger disconnects 12V power from the roof mounted lights and fan.
On my A-frame, the switch is hidden behind the curtain that hides the front roof hinge in the corner I mentioned. Here are pics with curtain in place, and curtain removed and switch showing. When I upload some pics, the software rotates them 90 degrees counter clock-wise.
2014 Rockwood A122 A-frame