I have a 2018 30DS. Last trip someone pointed out a switch under the TV Cabinet over the sink. I vaguely remember it being explained on the walkaround. I haven't used it so promptly forgot what the guy said.
What the heck is it for?
That switch turns on all the ceiling lights in the front part of the motorhome. There is no set-up to be done, it just turns on all those lights. The lights over the sink, couch and dining table are separately controlled by the little switches on each light. At least that is the way it is on my 2018 30ds.