Most RPCVs who completed their service relatively soon after college would qualify for GS-5, maybe GS-7. GS-7 usually requires at least 1 full year of experience that nearly matches the exact responsibilities of the job posting. So, unless you have a Masters or coincidentally worked for a year doing the same thing at a different place, RPCVs start out at GS-5. There is one exception, though. If you had a particularly good GPA from your undergraduate studies (like a 3.9/4, or something like that), that bumps you up to GS-7. GS-9 is really ONLY applicable to people with a Masters or equivalent higher degree.
Also, make sure you research the appropriate format for a federal resume. It is VERY different from a standard one- or two-page resume. Keep in mind that submitting your resume to NCE-only positions might still result in a 6-month wait to receive an interview; at least, that's been my experience.