Jason, here is a privy dig me and my daughter did last spring, south of Topeka 30 miles. The easiest way to find them is with copper dowsing rods, then verify with a spring steel probe. Dowsing is the only way to go! Probing gets tiring after an hour! I've also got a dowsing video on my youtube site, its about finding graves.
Privy digging http://www.youtube.com/watch?v=8v47qaOHTQE
Privy digging http://www.youtube.com/watch?v=8v47qaOHTQE