Pull the fuse.
Measure Ohms (2000 / 2k ) between the wire going from the switch to the light. That would be either end of the same wire, you should read close to 0 ( up to about 2 could be test leads) then if that is good measure across the light socket that should be 0L. Or open. ( digital / analog meter) If it is not ( 0L ) the wire from the switch to the light is the problem. If that is the case if you have not already pull the light and look at the wires on the back, if you get lucky there will be a problem with the wire there.
Again fuse is pulled and wire is off switch.
2015 F350 Lariat Diesel Dually, White, Hitch Kit.
2013 Dutchman Voltage 3200 Epic II 5th wheel.