If you are traveling on interstates, the download an use either of these apps: iExit or iExit Trucks. The truck version shows rest stops (with facilities) and trucker rest areas (no facilities). I like it better for finding truck stops for diesel fuel.
Not on interstates? A good paper state highway map usually shows rest parks, which are like "rest areas".
Check each state regulations on staying overnight in the rest areas. Some states allow it, some do not. Also, while in general Walmart permits overnighting at manager's discretion, many cities and towns prohibit it.
Personally, I use RVTripWizard to plan a route and look for campgrounds along the way. I like reserving ahead because as I read somewhere recently: "Thousands of RVs are being sold every year, but number of new campgrounds opening up every year is small."
Tom, Kathy & 2 Goldendoodles, Sandy & Lucy
2017 Thor Citation Sprinter 24SS
2019 Newmar Ventana LE 3412, Freightliner XCR, Cummins ISB6.7, 2014 Honda CR-V FWD