Your interior lights are all powered by your COACH battery system.
At times, when your battery is not fully charged, one longer run if wiring may have just enough loss for your lights at the end of that run to operate properly.
So, assuming you do not have any damage to your wiring, and assuming that you have checked that all of your fuses on your interior 12-VDC fuse panel are all OK, then you should check your COACH battery. Does it have water? Is it relatively new? Are all of the connections on your COACH battery in good condition and firmly mounted on the battery terminals? and finally, is your battery fully charged. You can check this with a multimeter and you should read across the battery terminals and you should read around 13.5-VDC if fully charged, around 14.5-VDC if the battery is being charged. A reading of around 12.5-VDC means that your battery is not charged and your Converter may not be operating properly.