[Richard]

Let’s start with the plug on the coach. Using your voltmeter, do you see any voltage at the plug when the brake, headlights, or turn signals are activated?

The next place to look will be in the engine compartment. Newell typically uses a relay based system for the rear plug. I don’t know exactly where they would be on your late model coach. But find those relays, and using a voltmeter determine if they are getting signal voltage, and supply voltage.

And another thing that goes with NO toad lights, is grounding. Without a good ground, nothing will work.

Did they work, and stop suddenly? Or is this a problem that has always existed?

Sorry for all the silly questions, but the armchair mechanics like me are only as good as the info provided. We don’t know what you know about the situation.