Many Camping World stores, Flying-J and so on have dump stations, often for a fee, Plus there is a smart phone app RV-Assistant as I recall that lists dump stations, Many (If not most) parks will let you dump for a fee, Usually 5-10 bucks. even if you are not staying there.
As for rest areas. Many states that had them (IE: GA where I'm stuck) have closed them due to vandals. I won't go into all the damage done, but Well, Frankly I find some of it hard to believe.. If, that is, i had not seen proof.