Is there such a thing as a low RPM alternator

Alternators do vary. Buying a larger output (amperage) is not necessarily better for lower RPM. In fact, in some cases, lower output alternators put out more at low RPM than the larger model.

Really, the only way to know for sure is to "find" (good luck) the engineering specs. Years ago, we actually had some Delco books which had graphs of shaft RPM vs output.

Generally, in "stock" Mopars, the squarebacks are better.

I don't know about the newer Denso, other stuff. I've been told some of the Ford alternators are very good.

You will NOT, however, find one that puts out "full capacity" at low RPM.

What are you running for idle speed, pulleys? IE small crank pulley? large alternator pulley? How about your loads? You running electric fans?