I went out and cleaned the roof today, and it looks a little better. I have cleaned it at least twice a year with Camco Rubber Roof Cleaner/Conditioner and my soft RV wash brush. Maybe I cleaned it too much? Also, I think the cover (which is breathable) allowed mold or whatever to grow, and that's what wrecked the top white layer of the roof. What's remaining feels solid, but just looks bad.
So, I think I'll lightly reapply the Dicor and roll on some Henry Tropi-Cool 100% Silicone coating. Does that sound like a good plan?