I have crisscrossed the country in my orange '74 many times over the last few years, and have camped just about everywhere imagineable - except for paysites. The key for me is good maps (DeLorme's Atlas and Gazateer), that allow me to get far enough off the paved roads, on public lands of course, to not be seen by police, rangers, or locals driving the highway. I always properly dispose of my waste, and only have a small fire if a fire ring is previously there. Only twice, have I been awoken by law enforcement apologizing for waking me, but saying that it is their job to make sure everything's OK.
Walmarts work too, but you are supposed to be fully self contained. When stuck in a town without a Walmart type option, I've found that trying to hide only draws suspicion. Instead, I wait until bed time, drive to a previously selected spot where other cars will be parked overnight, crawl into bed without shutting the curtains (suspicion, again), and get up and leave before anyone else in town awakens.
Asking law enforcement before setting up camp has worked well too, if I can find them. I was once even allowed to sleep in a pullout on Hwy 1 in Marin - just because I'd asked first, he would've otherwise made me move on. Ranger's and police will often ignore 'no camping' rules if they know whom they are dealing with. Some of the best sites I've found were recommended by National Forest Rangers.
Happy travels,
Jeff