When I dim the interior lights, the RioCar display stays at the same brightness. Is this the correct behavior?

Keep in mind it's a headlight sense and not dimmer sense. As long as there is a difference in the voltage when your headlights are on and off on the current wiring, it should be fine. To adjust it, open the settings menu, pick dimmer, and set it. Once finished, turn on your headlignts and do the same thing. Once you have it set, turning on and off your headlights should switch between the two settings.