[jimcol]

I had your same experience years ago on my 2006 when installing a back up camera. Had to change all the running lights to LED bulbs and that solved my issue. May not be your issue as you said you saw the drop at the fixtures with all the bulbs pulled. Installed a back up camera for my neighbor on their fifth wheel. Instead of tying it into the running lights we picked up 12 volts off an interior light circuit in his coach. Easy wire fish as his switch was on the rear wall. Ran it to a 12 volt remote relay. He could operate the relay from his truck.