I hold two passports, American and French. My french passport expired in 2003. I would like to renew my French passport but am not sure that I can produce any meaningful French identity documentation other than my expired French passport.

What do I need to do to renew the expired French passport?

In most countries you don't lose citizenship unless you specifically renounce it, if you had a french passport then you were a french citizen. Just contact the French embassy in the US (I assume you are in the US now) and they should renew it.