Yay, somebody's doing Peace Corps! Good for you. That's #1 on the list of things I wish I'd done when I was younger. (I don't have years to burn before med school anymore.)
It takes 6-18 months to get your PC assignment - have you started the application process?
One of the biggest assets in doing a PC gig is the language you'll learn. You might have some choice in which language: think about what you might want to do with it later. Spanish and Chinese are more useful within the US, for instance, and French is more useful if you want to be involved in Africa or with the UN/WHO/MSF.
Also look into Masters International programs, where you can combine PC service with an MPH or other masters degree. This doesn't help you get your premed prereqs done, but some graduate work is never a bad idea when you have a weak undergrad GPA.
As for your 3.0 and what you do next academically, I think that waiting to do your prereqs until after you get back is a better plan. You might discover during service that medicine isn't the contribution you want to make. If medicine persists as a goal, then I recommend getting some distance between you and your 3.0. You'll be a different person when you get back.
In theory. if you take your prereqs before you start service, you could be studying for your MCAT during service, but I don't think this would really happen. You won't want to be carrying a coffin full of books around, and you won't have internet access.
Whenever and however you go back to school to do your prereqs, you absolutely must demonstrate that you're not "a 3.0." I strongly suggest that you apply to a structured postbac program, such as Bennington or Mills, when you return from service. This sets you up to succeed GPA-wise as well as on the MCAT.
Best of luck to you, and do keep us posted.
