I've used highway rest areas, Walmarts, Cracker Barrels, and city streets many times with an RTT, and now a pop-up wedge camper (GFC). City parks are often off limits for overnight camping, but there are some that allow it. It works better with the GFC, since I can fully button up the truck bed and camper, and if there's ever a problem, it takes less than a minute to pack up & leave. I've never had that happen, but it's reassuring to know you can be underway before most people realize you're gone.
I use FreeCampsites.net & iOverlander for unscheduled landings. Works well for me.