Most Wal-Mart stores do allow overnighting in their lots (not saying we have done this... more than 1,000 times, that is!). We have found The Next Exit guidebook (what's at all of the exits on all American Interstates) to be very helpful in locating camping areas, Wal-Marts and truck stops. You can order it on-line or pick it up a an RV dealer. As moosetags says, truckstops are good too, if you can handle the truck generators running all night. Also state welcome centers and rest stops have truck/RV parking and a lot of them have RV dumps as well. It tells you which ones in The Next Exit.
Good luck, and happy camping!