Not knowing the RV, I made some guesses in looking at drawings, so need you to check my thinking??? The drawings show items but often not locations, so I am guessing that you have three lights that come on at the same time with one switch? Correct me if that is not true!!!
BUT if that is correct and these are the lights in question, they are shown as working on the same wiring circuit!
This snip of drawing shows wire PH as power coming in from the switch to all three lights and then out on ND to ground to complete the full circle or circuit.
Appears these three work off the same switch!
IF we are speaking of the correct lights, I would then go to each of the lights which work, pull it out and look for loose wires that feed on down the line to the light which does not work.
2015 Winnebago Vista 31KE on 2014 chassis