It's possible they have 2 switches and both have to be on.
In my 19RD, there is a switch by the door that powers up all 4 ceiling lamps that run the length of the TT. Each fixture has it's own on/off switch.
We leave all 4 individual switches on and control the whole bank from the switch by the door.
So, make sure all the mystery lamps have their switches ON while trying various wall switches.
TT 2015 19RD "TheJayco"
TV 2003 F-350 "Montblanc" - Housebroken chore truck
Sitting in The Cheap Seats.
And proud of it!