I mean, there are American values. The country was built around the idea of limiting the federal government's power, and adherence not a person, but to a set of values (like freedom of speech). We're explictly founded on the concept of rule of law.
Through that lens, it is very clear what anti-American means: adherence and loyalty to a person instead of a set of values, expansive government overreach, erosion of individual liberties, including the ability to express dissent.
What Trump and his followers mean by it, though, is that any disagreement with Trump on policy, strategy, tactics, procedure, or belief is anti-American. The way they use the term "anti-American" is anti-American. Therefore the term outside of context is meaningless. It's been coopted, twisted, and perverted.
It was also built around a process by which those values can be codified and changed to reflect changing times, i.e. the amendment process. Which we have forgotten about entirely.