David,
I would confirm that the ground of your test light is a known ground. (a frame mounted bolt). The coach came with a 6 pin plug originally in the mud flap. It was configured with a ground, a 12volt charge wire, right and left turn, a brake control wire, and running lights. You may want to check the connections in this plug because the mud flap takes a lot of abuse back there from movement. If you trace that wiring harness from the plug, you will see where it is connected to the main harness on the right side of the engine. Check that location to see if that was modified. Check the fuses up in the front run panel (first compartment on the drivers side in the front). Hope this helps, Fred