The third switch is likely an interior ceiling light, in the living room area. The lights have a switch on them too, so turn that switch on, then turn on all the interior lights in the trailer, then turn the switch off. It should turn off one of the ceiling lights. Just leave the switch on the light itself on all the time, so the wall switch operates it.
2007 T-286SR Cherry/Granola, #6236, original owner, current mileage: 9467.8 (as of 5/26/19)
1997 T-2653 Blue Denim, #5471
1979 12 1/2' MC, Beige & Avocado, #4639
Past Sunlines: '97 T-2653 #5089, '94 T-2251, '86 T-1550, '94 T-2363, '98 T-270SR