Short answer? It depends.

Rest areas are operated by the individual states, and the rules vary widely.

Signs at almost all rest areas will say "no camping," but experienced boondockers are not looking for a camping experience at a rest area. They just want to park in a safe place off the road, have a bite to eat, and get some sleep before heading off on the road again.

So, yes, you can legally boondock at rest areas, but it depends on where you are, what your RV is like, and what you consider "staying overnight."