That large, white motorhome dashboard is an eye-killer when driving with the sun ahead of you, and you can sure feel the reflected heat. At night, it sometimes causes annoying, even hazardous, bright reflections on the inside of the windshield.
I suppose, you could paint it black, which would cure the reflections, but then the dashboard would turn into a blast furnace on sunny days. The simplest cure is a black, or very dark-colored, dashboard mat -- it absorbs both the light and the heat, without re-radiating it.
You can easily make one from a dark carpet remnant, or buy an eight-foot length of hallway carpet runner at Home Depot, then trim to fit. If you haven't done something like this before, especially if your dash is not a flat slab, make a template out of heavy wrapping paper to follow the contours, then cut out the carpet. Stick-on Velcro tape, or a few well-placed stainless-steel snaps, will keep it from sliding around.