Free camp sites.net-compendium, google earth, are sites to start with. Also, we find youtube useful for ideas on travel. Check Cabelas for some stores have good overnight parking--also Cracker Barrel has tight parking. Use Google Earth to check out the possible stops.
The Walmart parking trend is being stamped out in some places due to free loaders and "full timers" staying in the lot for days and weeks.
There are even "rest areas" that have signs reading no overnight parking. I'm not a trouble maker, but what is a rest area other than to be a place to rest?
As a last resort, truck stops can be used. We try to avoid them due to the fact that truckers are having to obey the log book laws more closely now and the truck stops are crowded. Plan on stopping before 3 P.M. if you can to guarantee a spot.
We're not cheap, but do try to avoid high dollar RV parks when just going from place to place--we've not had too much trouble getting a stop for the night in most cases. We also try to drive large cities later at night or about 4 A.M. to avoid crowds.
