The Rambam does not hold you can perceive moral values. The Rambam holds there is Natural law --but it can't be perceived. It has to be known through revelation like with Avraham Avinu [Abraham the Patriarch]. And later on there was a higher degree of revelation at Mount Sinai. But both Natural Law and Torah Law need to be revealed.
Many post-Enlightenment Jewish and Christian thinkers resented the effort of the Middle Ages to integrate reason and revelation and degraded the role of reason in the determination of moral values. But this backfired. I will not go into all the post Enlightenment philosophers that were only too happy to relegate reason to figuring out that bachelors are not married. I will not go into the disastrous linguistic and so called analytic "philosophy" of the twentieth century and terrible totalitarian philosophies like Feminism, Nationalism, and Communism and the American Supreme Court. But let me just say that I think throwing out the great philosophers of the Middle Ages was a disaster.
I want to mention that I hold from both the position of Maimonides This middle position seems to me to be where Maimonides is. The first plane of knowledge is immediate first principles. But it is perceived in some third type of way. And then and after that comes the Kantian synthesis, where understanding allies concepts of pure reason to a priori objects and to empirical objects. (This is called Daat by Isaac Luria and Shalom Sharabi.)
What I am trying to say is that moral values have two parts to them. There is the internal principle--the thing in itself (the dinge an sich.) That is not accessible to human reasoning or perception. Rather to non intuitive immediate knowledge. The other part is the applications to specific situations. This aspect of moral is what is called "universals". Can be understood to apply to moral just like they can to other areas.