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The issue is not the RealField. The issue seems to have something to do with the precision. 28.0 is simply a RealNumber with the default precision of 53 bits. Here are my results for various precisions:

sage: erfc(RealField(53)(28))
6.56321584032878e-343
sage: erfc(RealField(64)(28))
6.56321584032878415e-343
sage: erfc(RealField(128)(28))
6.5632158403287841523809104816062747741e-343
sage: erfc(RealField(192)(28))
6.56321584032878415238091048160627477413416268994828261802e-343
sage: erfc(RealField(256)(28))
uceil_log2.c:40: MPFR assertion failed: exp < 1023
---------------------------------------------------------------------------
RuntimeError                              Traceback (most recent call last)

The real error is the "MPRF assertion failed: exp < 1023". This probably has something to do with the C implementation of mpfr_erfc, which is a third party library. I.e., it is used by SAGE, but it was not written by the SAGE team. I believe the MPFR library is still actively maintained, so maybe the SAGE team can reach out to the MPFR developers?

sage: erfc(RealField(53)(28))
6.56321584032878e-343
sage: erfc(RealField(64)(28))
6.56321584032878415e-343
sage: erfc(RealField(128)(28))
6.5632158403287841523809104816062747741e-343
sage: erfc(RealField(192)(28))
6.56321584032878415238091048160627477413416268994828261802e-343
sage: erfc(RealField(256)(28))
uceil_log2.c:40: MPFR assertion failed: exp < 1023
---------------------------------------------------------------------------
RuntimeError                              Traceback (most recent call last)

Edit: By the way, if you want to test with a standard double precision floating point number, put a letter 'r' after the number:

sage: isinstance(28.0, sage.rings.real_mpfr.RealNumber)
True
sage: isinstance(28.0r, float)
True
sage: erfc(28.0)
6.56321584032878e-343
sage: erfc(28.0r)
0.0