Installing a dimmer switch

Need some expert advice to install a dimmer in my Grand Design Reflection 31MB. The 12v cabin lights in the ceiling are way too bright. The switch is a blue lighted switch in the main panel in the hallway. I want to install the new on-off/dimmer switch. I can easily run wires in the wall to a more convenient location about 3 feet away. Behind the main switch is 2 red wires (constant power to the switch and one that goes to the lights when the switch is turned on - see pic. The 2 white wires plugged into the current switch are probably ground?
I thought I'd just eliminate the main switch in the control panel and use the new switch for on-off and dimming.
How the heck do I wire this up?