This is usually for the overhead lights.
I had a 1995 Coleman Royale PUP that had the same switch. I always found it frustrating that I couldn't turn the lights on to see what I was doing during setup without putting the stove up.
It always seemed that each time we set that camper up it was either dark or raining.
2011 Silverado CC/SB
2014 Cougar 31SQB