You said that you do not have a multiplex system, but I can't imagine a Marquis of that model year not having that type of lighting control system. If your light switches are the old style rocker switch, I would agree with you. If they are a keypad with four, six or ten buttons on it, I would expect a PLC control system such as Intellitec.
If so, try pressing any of the buttons for that light circuit and hold the button for a couple of seconds. On ours, pressing the button turns the lights on, if they're off, and off, if they're on. If you press it and hold it for more than a second, or so, it will either start incrementing brighter or dimmer, depending on what was done the last time. Holding it will sequence through the brightening and dimming in a continuous sequence as long as you hold the button.
I'm sorry you're having this problem, but do wish you luck and great enjoyment with your coach.
Jim & Angie, Boise, ID
2021 Entegra Esteem 27U
2017 GMC Acadia Denali