The most likely culprit is the fuse.
But unfortunately that is not necessarily the case since every RV has it's own particular quirks. You may have to check all of the bulbs. It could be like an old fashioned string of Christmas lights where if one goes out, then they all go out. It could also be a case where, if one light burns out the rest stay on, BUT if one the bulbs is loose or removed, then the whole circuit quits. I had a situation in an automobile where, if one taillight burned out, it automatically burned out the other one as well. They always burned out in pairs. I never did figure out why.
Hopefully it is not case of a mouse eating off the insulation and causing a short circuit.