I don't know about the Phaeton's taillight pricing specifically, but usually RV lighting selected from commonly available light fixtures used on automobiles. If you google the numbers that are molded into your light fixtures you should be able to determine what vehicles use that same light fixture.
As example, the rear lights used on our Allegro Bus are the same as used on a Ford Ranger and are relatively easy to replace or upgrade.
Joe & Shelly, Justin, Tyler, Alyssa | Butler PA
2008 Tiffin Allegro Bus 43QRP|Cummins 425|Honda CRV