We have 2017 Chateau 31W and had same issue with ceiling lights. After viewing 2 YouTube videos by RVProject (all about 12v dimmers) and (12v dimmer shootout), learned that I had to use a 3 wire high side dimmer because Thor did not use a single white 12v common wire that only fed the ceiling lights. A 4 wire dimmer requires that you insert the dimmer in both plus and common wires going to the lights. I bought a AH-SLD-5-HS01 slide dimmer with switch ($30) from pdxrvwholesale.com which has same form factor as the double switch controlling the ceiling and sofa lights. Dimmer for in same hole and existing ceiling switch wires connect to switch on dimmer. Connected third wire from dimmer to any one of the white common wires running behind the switch. Also bought new single switch from pdx and cut new hole for single switch and connected existing wiring from old sofa light switch to the new single switch. DW very happy with ability to dim ceiling lights.