Probably late for your trip, but I recommend the Free Campground Guides-East and West editions. Got mine on Amazon.com. They have a good listing of free and low cost (under $12) places like small town parks and local recreation areas. Don't expect much in amenities, but we have found some delightful surprises. Some of the places have a drop box for a nominal contribution. We have found nicely kept parks with free electricity and dump stations in some midwest towns. Others are not so nice, but still a place to park. Another trick that has worked on occasion is to hunt up the local police and ask where a good place to park is. We have been invited to the city parking lot (Plymouth, MA) and the police parking lot (somewhere in NE). If you are not welcome, they will let you know (e.g., Flagstaff, AZ). Wal Mart and the like are usually too busy and bright for our tastes unless the last "resort".
'09 Pleasure Way Lexor, Chevrolet Express 3500 chassis, 6.0L V8
Last RV, 2005 NRV Dolphin 5342 (2007-2014)