To add to what Jim said, I don't think you will have much luck finding
an RPG to C translator.  The few cross-language translators I have seen
seem to provide only RPG to COBOL conversion, since the two languages
share the most in common.  If C is an absolute requirement, I'm afraid
you'll have to do it yourselves.
 
I seem to remember a company thst specialized in cross-translators:
Metaware?  They featured a slew of cross-translators for most everything
you could imagine, including Autocoder to COBOL!  One used to find their
ads in the Byte deck.
 
--Jim