We generally plan to travel only 250 a day and we have done an overnight in Walmart on several occasions. Most states prohibit overnight stays in their rest areas. We have had generally had good luck with Walmart. Sometimes local laws will not allow you to camp overnight, so we always call ahead to make sure it is OK and where they want us to park.
We sometimes make reservations and sometimes we don't. If the park we are headed to is very popular and fills up quickly we make reservations.
BTW we have been fulltimers since June 2011.