This can make things hard.  I would probably check three things.  One would be the USPS website and see if the rural route numbers still attached to any regular addresses using their conversion page.  Secondly, I'd use the GPS coordinates in online mapping program (GoogleMaps, YahooMaps, Mapquest, Thomas Brothers, RandMcNally) to get a satellite view of the lots and see if any address is listed at that site, plus attempt to nail down adjacent sites.  Finally, I'd coordinate any and all data with the online Assessors Parcel Map, if available.  I would do this, and then see about going into the Records Offices to view the other related maps (Subdivison, survey, parcel, etc...). 

I've found that this usually works in about 85% of the cases.