class c upper clearance lights

They go in sections of pos wire around the top of the Rv. If one light in that section loses its pos wire connection the whole section goes but the other sections work. Pull each light in that section that does not work to find the "open".

All other lights work

Has the water resistance been inspected and repaired if necessary?

They are in the parking light circuit have you checked to see if your chassis parking lights work? If not, I would check the parking light fuse for beginners.