The RETS standard doesn't include support doing that. The field sets for classes could be the same or could be have drastically different definitions. Also fields can have different lookup values assigned to different classes.
To determine the class of the listing Paragon RETS returns the L_Class field. Depending on whether you are using COMPACT or COMPACT-DECODED it will return either the class numeric value or the description of the class.
COMPACT = L_Class=1
COMPACT-DECODED=RESIDENTIAL/FARMS
Currently in paragon the numeric value is appended to the classname value ie. RF_1.
To find the class using the value returned in COMPACT-DECODED you would need to lookup the description in the class metadata:
<class>
<classname>RF_1</classname>
<description>RESIDENTIAL/FARMS</description>
...
</class>
We do not have a field that returns the className value directly.